Every season of Crash Course Biology ever, ranked from best to worst by thousands of votes from fans of the show. The best seasons of Crash Course Biology!

And thus begins the most revolutionary biology course in history. Come and learn about covalent, ionic, and hydrogen bonds. What about electron orbitals, the octet rule, and what does it all have to do with a mad man named Gilbert Lewis? It's all contained within.

That's Why Carbon Is A Tramp

Hank teaches us why water is one of the most fascinating and important substances in the universe.

Water - Liquid Awesome

Hank talks about the molecules that make up every living thing - carbohydrates, lipids, and proteins - and how we find them in our environment and in the food that we eat.

Biological Molecules - You Are What You Eat

Hank tells us about the city of Eukaryopolis - the animal cell that is responsible for all the cool things that happen in our bodies.

Eukaryopolis - The City of Animal Cells

Hank describes how cells regulate their contents and communicate with one another via mechanisms within the cell membrane.

In Da Club - Membranes & Transport

Hank describes why plants are so freaking amazing - discussing their evolution, and how their cells are both similar to & different from animal cells.

Plant Cells

In which Hank does some push ups for science and describes the "economy" of cellular respiration and the various processes whereby our bodies create energy in the form of ATP.

ATP & Respiration

Hank explains the extremely complex series of reactions whereby plants feed themselves on sunlight, carbon dioxide and water, and also create some by products we're pretty fond of as well.

Photosynthesis

Hank and his brother John discuss heredity via the gross example of relative ear wax moistness.

Heredity

Hank introduces us to that wondrous molecule deoxyribonucleic acid - also known as DNA - and explains how it replicates itself in our cells.

DNA Structure and Replication

Hank imagines himself breaking into the Hot Pockets factory to steal their secret recipes and instruction manuals in order to help us understand how the processes known as DNA transcription and translation allow our cells to build proteins.

DNA, Hot Pockets, & The Longest Word Ever

Hank describes mitosis and cytokinesis - the series of processes our cells go through to divide into two identical copies.

Mitosis: Splitting Up is Complicated

Hank gets down to the nitty gritty about meiosis, the special type of cell division that is necessary for sexual reproduction in eukaryotic organisms.

Meiosis: Where the Sex Starts

Hank guides us through the process of natural selection, the key mechanism of evolution.

Natural Selection

Hank explains speciation - the evolutionary process by which new biological species arise - in terms of finches, ligers, mules, and dogs.

Speciation: Of Ligers & Men

Hank discusses the process by which organisms grow and develop, maintaining that, in the end, we're all just tubes.

Animal Development: We're Just Tubes

Hank introduces us to the relatively new field of evolutionary developmental biology, which compares the developmental processes of different organisms to determine their ancestral relationship, and to discover how those processes evolved.

Evolutionary Development: Chicken Teeth

Hank talks about population genetics, which helps to explain the evolution of populations over time by combing the principles of Mendel and Darwin, and by means of the Hardy-Weinberg equation.

Population Genetics: When Darwin Met Mendel

Hank tells us the background story and explains the importance of the science of classifying living things, also known as taxonomy.

Taxonomy: Life's Filing System

Hank gets real with us in a discussion of evolution - it's a thing, not a debate. Gene distribution changes over time, across successive generations, to give rise to diversity at every level of biological organization.

Evolution: It's a Thing

Hank introduces us to comparative anatomy, which studies the similarities and differences in animal anatomy to support the theory of evolution and the shared ancestry of living things.

Comparative Anatomy: What Makes Us Animals

Hank introduces us to the "simplest" of the animals, complexity-wise: beginning with sponges (whose very inclusion in the list as "animals" has been called into question because they are so simple) and finishing with the most complex molluscs, octopuses and squid. We differentiate them by the number of tissue layers they have, and by the complexity of those layers.

Simple Animals: Sponges, Jellies, & Octopuses

Hank continues our exploration of animal phyla with the more complexly organized annelida and arthropoda, and a biolography on insects.

Complex Animals: Annelids & Arthropods

Hank introduces us to ourselves by taking us on a journey through the fascinatingly diverse phyla known as chordata. And the next time someone asks you who you are, you can give them the facts: you're a mammalian amniotic tetrapodal sarcopterygian osteichthyen gnathostomal vertebrate cranial chordate.

Chordates

Hank and his cat Cameo help teach us about animal behavior and how we can discover why animals do the things they do.

Animal Behavior

The Nervous System

Hank takes us on a trip around the body - we follow the circulatory and respiratory systems as they deliver oxygen and remove carbon dioxide from cells, and help make it possible for our bodies to function.

Circulatory & Respiratory Systems

Hank takes us on the fascinating journey through our excretory system to learn how our kidneys make pee.

The Digestive System

The Excretory System: From Your Heart to the Toilet

Hank introduces us to the framework of our bodies, our skeleton, which apart from being the support and protection for all our fleshy parts, is involved in many other vital processes that help our bodies to function properly.

The Skeletal System: It's ALIVE!

Hank tells us the story of the complicated chemical dance that allows our skeletal muscles to contract and relax.

Big Guns: The Muscular System

Hank tells us about the team of deadly ninja assassins that is tasked with protecting our bodies from all the bad guys that want to kill us - also known as our immune system.

Your Immune System: Natural Born Killer

Great Glands - Your Endocrine System

The Reproductive System: How Gonads Go

Hank veers away from human anatomy to teach us about the (mostly) single-celled organisms that make up two of the three taxonomic domains of life, and one of the four kingdoms: Archaea, Bacteria, and Protists. They are by far the most abundant organisms on Earth, and are our oldest, oddest relatives.

Old & Odd: Archaea, Bacteria & Protists

Hank introduces us to nonvascular plants - liverworts, hornworts & mosses - which have bizarre features, kooky habits, and strange sex lives. Nonvascular plants inherited their reproductive cycle from algae, but have perfected it to the point where it is now used by all plants in one way or another, and has even left traces in our own reproductive systems.

The Sex Lives of Nonvascular Plants: Alternation of Generations

Hank introduces us to one of the most diverse and important families in the tree of life - the vascular plants. These plants have found tremendous success and the their secret is also their defining trait: conductive tissues that can take food and water from one part of a plant to another part. Though it sounds simple, the ability to move nutrients and water from one part of an organism to another was a evolutionary breakthrough for vascular plants, allowing them to grow exponentially larger, store food for lean times, and develop features that allowed them to spread farther and faster. Plants dominated the earth long before animals even showed up, and even today hold the world records for the largest, most massive, and oldest organisms on the planet.

Vascular Plants = Winning!

Hank gets into the dirty details about vascular plant reproduction: they use the basic alternation of generations developed by nonvascular plants 470 million years ago, but they've tricked it out so that it works a whole lot differently compared to the way it did back in the Ordovician swamps where it got its start. Here's how the vascular plants (ferns, gymnosperms and angiosperms) do it.

The Plants & The Bees: Plant Reproduction

Death is what fungi are all about. By feasting on the deceased remains of almost all organisms on the planet, converting the organic matter back into soil from which new life will spring, they perform perhaps the most vital function in the global food web. Fungi, which thrive on death, make all life possible.

Fungi: Death Becomes Them

Hank introduces us to ecology - the study of the rules of engagement for all of us earthlings - which seeks to explain why the world looks and acts the way it does. The world is crammed with things, both animate and not, that have been interacting with each other all the time, every day, since life on this planet began, and these interactions depend mostly on just two things... Learn what they are as Crash Course Biology takes its final voyage outside the body and into the entire world.

Ecology - Rules for Living on Earth

Biology is the study of life—a four-letter word that connects you to 4 billion years worth of family tree. The word “life” can be tricky to define, but a shared set of characteristics helps biologists identify living things. In this episode of Crash Course Biology,  you’ll learn how all of life is connected, and why studying biology can help us better understand ourselves and our relationship to all living things.

Introduction to Biology

Science offers a way of discovering and understanding the world around us, driven by questions and tested with evidence. And it’s a twisty-turny team effort— you won’t find many lone geniuses out there, or straight lines from hypothesis to conclusion. In this episode of Crash Course Biology, we’ll talk about the big picture of how scientific progress is made, from peer review to mathematical models, with some exploding eggs along the way.

The Scientific Method

A biologist’s natural habitat is anywhere questions about life are being asked—whether the subject is a nematode or a narwhal, a single cell, or a whole ecosystem. In this episode of Crash Course Biology, we’re flipping the microscope around to show how biologists’ work goes down. Along the way, we’ll learn why zebrafish and fruit flies are some of biology’s next top models.

What Biologists Do

Here on Earth, life is dizzyingly diverse—but it’s also surprisingly organized. A sense of order structures life and its processes, from the tiniest cell to the total sum of every living thing. In this episode of Crash Course Biology, we’ll uncover the levels of biological organization, discover soil’s superpowers, and find out why the biosphere is kind of like a really, really long train.

How Life is Organized

Ecology is the study of the interactions of living things with each other and their environment. It’s a field that not only lets us explore the interconnections between living things, but also how our environment affects us, and how we influence it in turn. In this episode of Crash Course Biology, we’ll get an overview of the field of ecology, see how matter and energy are conserved and transferred through ecosystems, and follow an ecological mystery surrounding the deaths of hundreds of elephants in Botswana, Africa.

Why Did All These Elephants Die? (Intro to Ecology)

Community ecology is the study of interactions between different species of living things, and lets ecologists examine the effects of predator-prey relationships, parasites, and mutually beneficial interactions. In this episode of Crash Course Biology, we’ll examine the myriad interspecies interactions with examples, see how keystone species impact their environment and explore how communities rebuild when they are disrupted, through the lens of the 1980 eruption of Mount St. Helens.

How Species Make and Break Friendships (Community Ecology)

When the Bald Eagle population started to decline in the mid-20th century, scientists began to ask why. Population ecology, the study of organisms of the same species, played a big role in answering that question. In this episode of Crash Course Biology, we’ll take a look at the methods population ecologists use to study a population and the types of data they collect. We’ll also find out how scientists helped bring the Bald Eagle back from the brink.

How Did We Save The Bald Eagle? (Population Ecology)

Life on Earth has weathered boiling-hot oceans and volcanic-ash-darkened skies—but that’s nothing like the climate change we’re experiencing now. In this episode of Crash Course Biology, we’ll talk about the greenhouse effect, learn why our climate is like a tangled pair of headphones, and discover that we’ve understood the science behind climate change for much longer than you might think.

What Is Climate Change?

Climate change shakes up all of Earth’s systems, including the living ones. In this episode of Crash Course Biology, we’ll see how climate change’s effects rattle the entire chain of life. Changes felt in one population ripple out to affect entire communities and ecosystems—whether they’re composed of pine trees, puffins, or people.

The Effects of Climate Change

Some scientists believe we are in the middle of Earth’s sixth mass extinction: a big, precarious game of Jenga that involves every ecosystem on the planet. In this episode of Crash Course Biology, we’ll see how conservation biology aims to restore habitat and preserve biodiversity. Along the way, we’ll see how environmental damage impacts human communities, and learn about wolves’ return to Yellowstone National Park.

How Do We Keep Life's Jenga Tower From Toppling? (Conservation Biology)

From a single-celled common ancestor, evolution has brought us all of Life’s Greatest Hits — including butterflies, beetles, bacteria, and human beings. In this episode of Crash Course Biology, we’ll learn how evolution explains life’s unity and diversity. Along the way, we’ll explore the fishy origins of a giraffe’s neck, and find out what a cat’s paw and your own arm have in common.

What a weirdly long giraffe nerve can teach us about evolution

Whether we’re talking about tigers, trees, or tarantulas, evolution happens at the level of the population. In this episode of Crash Course Biology, we’ll find out how natural selection, gene flow, genetic drift, and other processes drive changes in populations. We’ll learn about the Hardy-Weinberg equation, how your alleles make you uniquely you, and how some tigers changed their stripes.

Microevolution: What's An Allele Got To Do With It?

There are lots of ways that evolution happens, and natural selection is just one of them. In this episode of Crash Course Biology, we’ll find out how this process works and shapes traits in all living things —from ginkgo trees to howler monkeys. We’ll also learn how extra-grippy toes help some lizards survive hurricanes.

Natural Selection: Life's Way of Stayin' Alive

In this episode of Crash Course Biology, we’ll learn about the ways population genetics reveals how groups of living things evolve—by comparing genetic similarities and differences. We’ll discover the most genetically diverse species of all (hint: it’s not us), find out why “race” isn’t the biologically valid category we’ve made it out to be, and learn there’s much more in our DNA that we share than that sets us apart.

Why do we have different skin colors? (Population Genetics)

How can you tell two species apart? It’s not always simple. In this episode of Crash Course Biology, we’ll learn about speciation—a process that can happen over millions of years, or within a single generation. Along the way, we’ll discover how a single species can split into two and how a reptile from New Zealand continues to stump scientists.

Where Do Species Come From? (Speciation)

Humans may have been around for a long time, but life has existed for way longer. In this episode of Crash Course Biology, we’ll journey through deep time to uncover the history of life on Earth. We’ll explore the big, game-changing leaps where life diversified, changed, and just plain persisted.

How did life begin? (Evolutionary History)

Crocodiles, and birds, and dinosaurs—oh my! While classifying organisms is nothing new, phylogeny— or, grouping organisms by their evolutionary relationships—is helping us see life in a whole new light. In this episode of Crash Course Biology, we’ll learn why this kingdom-phylum stuff is going out of style and why phylogenetic trees are in.

How We're All Related (Phylogeny)

Everywhere you look on Earth, you’ll find wonderful and diverse living things, from tiny tardigrades to soaring sequoias. And incredibly, everything alive today, and everything that’s ever lived, is related. In this episode of Crash Course Biology, we reveal how the evolutionary relationships between living things define their place on a single, great Tree of Life, and we learn what that tree can tell us about our own place among the planet’s biodiversity.

Humans Develop Butt First (and other insights from the Tree of Life)

What’s a human? And how did we become humans, anyway? In this episode of Crash Course Biology, we’ll meet some of our closest relatives and trace how we evolved into the brainy, inventive, complex species we are today.

Humans Didn't Evolve From Chimps (Human Evolution)

Despite the diverse appearance and characteristics of organisms on Earth, the chemicals that make up living things are remarkably similar, often identical. In this episode of Crash Course Biology, we’ll look at the building blocks of the four major classes of biomolecules, how those join up to form macromolecules, and how a team of six atoms forms the vast majority of living matter.

What is Life Made of? (Carbon & Biological Molecules)

This is a love letter to water, life’s solvent, and one of the most wonderful molecules around. In this episode of Crash Course Biology, we’ll learn about how water’s polarity and hydrogen bonding help it sustain life on a larger scale. We’ll see how some water-based solutions can be acidic or basic, and examine how our bodies maintain the narrow pH range necessary for life.

A Love Letter to H2O: Water & pH

There’s an immense world of tiny stuff within us and around us—but how do we know about it? In this episode of Crash Course Biology, we’ll discover how we see what we can’t see, thanks to the help of centuries-old tools and more recent technology. Along the way, we’ll learn about the major types of microscopes and how to use a typical light microscope.

How We See What We Can't See (Microscopes)

The cell is the basic unit of life, and our understanding of it has advanced as science, and the tools available to scientists, has advanced. In this episode of Crash Course Biology, we’ll take a look at the difference between prokaryotic and eukaryotic cells, take a guided tour of the eukaryotic cell, and learn why most cells are small. We’ll explore the eukaryotic cell’s surprising beginnings through an endosymbiosis that occurred about 1.5 billion years ago.

A Tour of the Cell

The cell membrane is a protein-studded phospholipid bilayer that not only protects our cells, but also regulates what goes in and out. In this episode of Crash Course Biology, we’ll look at the structure of the bilayer, learn about its discovery, and explore the many ways substances can be transported into and out of cells.

How Does Stuff Get Into Your Cells? (Cell Membranes)

Even though it might seem like our bodies are on autopilot, there is a whole lot happening inside us to keep things moving. In this episode of Crash Course Biology, we’ll learn that our cells are in constant communication, reminding each other—and themselves—to perform important functions like breathing, walking, or even sleeping.

How Do Cells Communicate? (Cell Communication)

How Do We Get Energy? (Chemical Reactions)

How do cells get their energy? (Electron Transport Chain)

Photosynthesis: The Original Solar Power

Mitosis and the Cell Cycle

Why Are All Humans Unique? Meiosis

Why Your Cat Looks Like That: Genetics

Nature? Nurture? Not so simple: Genetic Traits

Our Instruction Manual for Existing: DNA Structure & Replication

How mRNA helped save lives: DNA Transcription

How RNA gets translated into protein power

If nearly all your cells have the same DNA, why are muscle cells so different from skin cells? In this episode, we’ll learn how gene expression is regulated in eukaryotes, and how methylating DNA, modifying histones, and messing with translation not only leads to different types of cells, but allows cells to adapt to the world around them.

How Genes Express Themselves

Science fiction is full of superpowered mutants, but in reality, mutations are much more diverse and complex. Sometimes, they can change someone’s entire body, and other times, we don’t notice them at all! In this episode, we’ll unpack what mutations are, how they work (including substitutions and frameshift mutations), and how scientists are learning to control mutations using tools like CRISPR/Cas9 and gene therapy.

Is drinking milk a Superpower? Genetic Mutations

Bacteria often get a bad rap, but they’re some of our best partners in science and medicine! In this episode, we’ll explore what bacteria are doing with their DNA — including how they can trade it around. We’ll learn about chromosomes and plasmids, gene expression and recombinant DNA, and how E. coli are used to make insulin.

We’re full of bacteria!

From the flu to COVID-19, viruses are a major threat in our everyday lives. In today’s episode of Crash Course Biology, we’ll learn why viruses are like genes in a box, and how they invade and spread between cells. We’ll also discover how vaccines and medicines help our bodies fight back.

How Do Vaccines Work?: Viruses & Vaccines

On its own, a huge DNA sequence is a meaningless pile of data — so, how do biologists figure out what it means? They turn to the power of bioinformatics! In this episode, we’ll learn what bioinformatics is, how it works, and how scientists have used it to better understand everything from evolution to a viral epidemic.

Algorithms Aren’t Just for TikTok: Bioinformatics

There are countless types of plants and animals on Earth, but how do they work? In this episode of Crash Course Biology, we’ll take a bird’s eye view of how multicellular life functions, including how it’s organized, how it regulates itself to maintain homeostasis, and the big question: Why are these living things so wildly complex?

Why We Aren’t Just One Big Cell: Multicellular Function

Plants may not seem like they’re doing much, but if you look closer, you’ll find a whole world just lurking beyond the surface. We’re talking chemical defenses, highways, and even ways to change the weather. In this episode, we’ll learn how plants get resources, get rid of waste, stay defended, govern themselves, and much more.

Plants Are Hardcore: Plant Anatomy & Physiology

Yep, this is the poop episode. Getting resources and getting rid of waste is so important, we have three whole systems dedicated to it! In this episode, we’ll learn how the respiratory system, digestive system, and urinary system work, and visit some other animals that process their resources in completely different (and sometimes totally wild) ways.

The Poop Episode: How Animals Turn Resources Into Waste

When you think about the body’s infrastructure, you probably think of bones. But what about the heart, the blood vessels, and the lymphatic system? In this episode of Crash Course Biology, we’ll tour the cardiovascular, lymphatic, and musculoskeletal systems, learning how all of them keep a vertebrate’s inner workings connected, powered up, and ready to move.

Why You’re More Than Goo: Animal Infrastructure

The world is full of microbes and viruses that can get us sick, but we’ve got an Avengers-style defense system ready to take them on. In this episode of Crash Course Biology, we’ll learn about an animal’s immune system, from their skin to inflammatory responses, to the adaptive immune system and all the special cells and antibodies that come with it.

How Skin, Snot, and Cells Keep Us Healthy: Animal Defense Systems

If cells and organs never talked to each other, an animal would fall apart faster than a boy band after the popular one goes solo. Thankfully, that’s where the nervous and endocrine systems come in! In this episode of Crash Course Biology, we’ll learn how they use neurons, hormones, and more to keep animals’ bodies coordinated.

What REALLY Happens When You Step on a Lego: Nervous & Endocrine Systems

When it comes to animal reproduction, there’s no one-size-fits-all strategy. Some animals need a mate, others don’t, and for some, it depends! In this episode, we’ll learn about sexual and asexual reproduction, internal and external fertilization, and all the ways that life, uh… finds a way.

How Animals Do It: Sexual & Asexual Reproduction

We tend to think life fits in two tidy categories: male or female, XX or XY. But it’s so much more diverse and complicated! In this episode of Crash Course Biology, we’ll discover sex and sexuality as a continuum across life. We’ll learn why humans are the only species with gender, and why same-sex mating is more common than you might think.

Gender, Sex, & Sexuality: What’s the Difference?

Why do animals do what they do? It’s a huge question, and in this episode, we’ll learn how scientists break it down. We’ll talk about the proximate and ultimate studies they use to understand animal behavior, and some of what they’ve learned along the way — including how much decision-making other animals are really doing.

Why This Toad is Bad at Jumping, and Other Mysterious Animal Behavior

Biology connects all of life—all of YOUR life. In the final episode of Crash Course Biology, Dr. Sammy is taking your questions. No matter how big, small, or personal, biology is in them all! He’ll show us how biology can help us solve problems and understand everyday life.

The Dr. Sammy Show

And thus begins the most revolutionary biology course in history. Come and learn about covalent, ionic, and hydrogen bonds. What about electron orbitals, the octet rule, and what does it all have to do with a mad man named Gilbert Lewis? It's all contained within. www.youtube.com/crashcourse

The Best Seasons of Crash Course Biology

The Best Seasons of Crash Course Biology

Season 1

Season 2