Test-Driven Development, or TDD, is a development practice in which tests are written before production code. The cycle begins with Red, writing a failing test that describes the desired behavior. The next step, Green, involves writing the minimum code necessary to make the test pass. The cycle concludes with Refactor, where the developer cleans up the code while keeping all tests green. A core rule is never to write production code without a failing test first, and each iteration is meant to be small, on the order of minutes rather than hours.
This Red-Green-Refactor rhythm produces several benefits. Because developers must specify behavior before implementation, TDD forces them to think about the API before diving into the code. Untestable code is often a signal of poor design, such as tight coupling or hidden dependencies, and so writing tests first naturally encourages SOLID principles, dependency injection, and small functions. The result is modular, loosely coupled code that is easier to maintain, extend, and understand. As a side effect, the test suite becomes living documentation, gives confidence when refactoring, and reduces the introduction of bugs.
Behavior-Driven Development, or BDD, extends TDD by expressing tests in natural language that describes behavior from a user's perspective. BDD uses an ubiquitous language shared by developers, QA, and stakeholders, and scenarios are typically structured as Given-When-Then. Tools such as Cucumber, SpecFlow, and Behave execute these human-readable specifications. The scenarios themselves are written in Gherkin syntax, which provides keywords like Feature, Scenario, Given, When, Then, And, and But to describe a workflow. Because BDD focuses on what the system does rather than how it is implemented, it complements TDD by aligning tests with business intent. The Given-When-Then pattern maps directly onto AAA: Given corresponds to Arrange, When to Act, and Then to Assert.