Key Differences Between TDD, BDD, and ATDD

When it comes to software development, building quality into the process from the very beginning is key—and that’s where approaches like test-driven development (TDD), behavior-driven development (BDD), and acceptance test-driven development (ATDD) come into play.

These methodologies help teams ensure that testing and quality assurance are embedded into the development cycle from day one, rather than being afterthoughts. But what sets them apart? While TDD, BDD, and ATDD share the goal of enhancing software quality, each has a unique focus, workflow, and team involvement. 

In this article, we’ll break down the key differences between these three approaches, explore their advantages, and help you understand when to use each.

Test-driven development (TDD)

Test-driven development (TDD) is a software development approach in which developers create automated tests before writing the code they aim to test. TDD is mainly centered around the development team, as its primary use is within unit tests, which are rarely created and executed by the testers.

TDD directs the coding through written tests, after which, once the coding is finished, the tests are executed and then refactored. The process follows a repetitive cycle called the Red-Green-Refactor. 

• Red phase. During this phase, the test is written by the developer, where the developer defines the expected behavior of a small piece of functionality. As the function is not yet developed, the test fails. 
• Green phase. In this phase, the developer is in the process of implementing improvements to the code to make the test pass. Once the functionality works as desired, the test can be considered as passed.
• Refactor phase. In this phase, the developer should not divert from the result achieved in the Green phase. Instead, the developer should work on improving the quality of the code, such as readability, structure, or even performance.

Developers might use frameworks such as NUnit, JUnit, PyUnit, or Cypress.

The end goal of TDD is improved code quality and correctness, which like the other two approaches contribute to ensuring the overall quality of the product.

Behavior-driven development (BDD)

Behavior-driven development (BDD) is a methodology that prioritizes collaboration among developers, testers, and business stakeholders to ensure the software meets the expected outcomes and behaviors. The product owner can also be a part of the BDD, transferring the requirements from the stakeholders to the QA team and the developers. This effort is aimed at creating a shared view of how the system should function from the user’s perspective.

The base of BDD is usually the syntax of the Gherkin language, which helps express the desired behavior of an application with test cases in a plain form that is easy to understand for the stakeholders. The Gherkin syntax consists of three main statements: “Given,” “When,” and “Then”. Each of the statements describes an aspect of the system. The syntax, however, is not limited to those statements only.

• Given statement. Sets the initial state of the system, providing the necessary context. This statement can be used multiple times to thoroughly express the scenario without having all of the preconditions in one sentence.
• When statement. The action or event that initiates the behavior being tested, is defined. This can be initiated by the system itself or the user.
• Then statement. Sets the expected outcome, which is a result of the initiation of the behavior under given circumstances.

Additional statements, as mentioned previously, include: 

• Feature. A high-level piece of functionality that is being tested.
• Scenario. A situation that is being executed within the tested piece of functionality.
• But. Keyword used to define conditions that limit or impact the previous step.
• And. Keyword is used to combine and chain multiple “Given,” “When,” and “Then” statements together.

The tool that helps with the execution of the tests is called Cucumber. It’s important to note that the tool executes the code linked to the corresponding steps in the test case, rather than the steps themselves. Because of the steps being linked to the code, traceability is supported.

The end goal of BDD is to create a shared vision of how the system functions, and foster collaboration between the business side and the technical side of the organization.

Acceptance test-driven development (ATDD)

In acceptance test-driven development (ATDD), a single acceptance test is written from the user’s perspective, focusing on meeting the system’s functional requirements. It addresses the critical question: is the system working as expected? This approach fosters collaboration among developers, users, and QA teams, uniting them around a shared goal of defining clear acceptance criteria. 

First, the team analyzes and discusses the existing user story and its acceptance criteria. After the analysis, they resolve any discrepancies, deficiencies, or defects. Based on the acceptance criteria and the analysis, the tester or the entire team begins creating test cases. Various test design techniques may be used, as multiple test types might need to be covered. The outcome is a test case that describes the desired behavior of the software. Stakeholders must clearly and easily understand test cases, closely aligning this approach with BDD principles.

The team typically starts by focusing on positive scenarios. If the system achieves the desired behavior without errors or exceptions and functions as intended, it proceeds to negative testing. Negative scenarios test how the system handles invalid or unexpected inputs. Finally, the team creates non-functional test cases, such as those for security, performance, and usability.

Unlike test-driven development (TDD), acceptance test-driven development (ATDD) prioritizes end-to-end acceptance criteria and uses natural language rather than the programming languages typically supported by unit-testing frameworks in TDD. As a result, wiki-like tools such as FitNesse are well-suited for implementation. When the acceptance criteria format allows for automation, tools like Robot Framework—an automation framework for acceptance testing—can be utilized.

The goal of ATDD is to verify functionality and deliver value to stakeholders.

The bottom line

Understanding the key differences between TDD, BDD, and ATDD is crucial for leveraging each approach effectively in software development. While TDD focuses on ensuring code quality at the unit level, BDD emphasizes collaboration and creating a shared understanding among stakeholders. ATDD, on the other hand, aligns development efforts with stakeholder expectations by focusing on acceptance criteria.

Each method plays a unique role in fostering a shift-left testing mindset and improving software quality early in the development cycle. By choosing the right approach for your project, you can streamline development, reduce errors, and ultimately deliver software that meets—and often exceeds—expectations.