Test Driven Development (TDD) is a way of writing computer programs. First, you write a test that tells the computer what you want it to do. Then, you write the code that makes it do what you want. Finally, you run the test and see if your code passed the test. If it didn't, you can adjust your code until it passes the test. TDD helps you make sure your code is working correctly and correctly doing what it's supposed to do.