What is Behavior-Driven Development (BDD)? Complete Guide

Parul Dhingra13+ Years Experience

Senior Quality Analyst at Deloitte

Updated: 1/22/2025

Behavior-Driven Development (BDD) is a software development approach that extends Test-Driven Development (TDD) by writing tests in plain language that describe how an application should behave from the user's perspective. BDD uses a structured format called Given-When-Then to create executable specifications that serve as both documentation and automated tests.

Understanding Behavior-Driven Development

Behavior-Driven Development emerged from Dan North's work in 2006. North was teaching TDD and noticed developers struggled with knowing where to start testing and what to test. He realized the word "test" caused confusion, so he replaced it with "behavior" and created a framework that described how software should behave.

BDD addresses three core problems in software development:

Communication gaps between business stakeholders and developers lead to misunderstandings about requirements. Business people describe what they want using domain language. Developers translate this into technical specifications. Translation errors cause incorrect implementations.

Documentation decay happens when written specifications become outdated. Traditional documents describe intended behavior at one point in time. Code changes while documents remain static. Teams stop trusting documentation that no longer reflects reality.

Requirement ambiguity creates different interpretations of the same specification. Written requirements often lack precision. Different team members understand requirements differently. These differences surface late in development when fixes are expensive.

BDD solves these problems through executable specifications. Teams write requirements in a structured format that both humans and machines understand. These specifications run as automated tests. When tests pass, the documentation accurately reflects system behavior. When tests fail, either the code or specification needs updating.

BDD is not primarily a testing technique. It is a collaboration method that happens to produce automated tests as a byproduct. The conversations that happen while writing specifications matter more than the specifications themselves.

The Three Amigos Meeting

BDD uses a practice called Three Amigos, where three perspectives examine each user story before development begins:

• Business representative (product owner, business analyst) explains what the feature should accomplish and why it matters
• Developer identifies technical implications, edge cases, and implementation considerations
• Tester questions assumptions, identifies scenarios that might fail, and considers how to verify the feature works

These three perspectives catch misunderstandings early. The meeting produces concrete examples that clarify expected behavior. These examples become the basis for BDD scenarios.

The Given-When-Then Format

The Given-When-Then format structures scenarios into three distinct parts:

Given establishes the initial context. It describes the state of the system before the action occurs. This includes user authentication status, existing data, system configuration, or any precondition needed for the scenario.

When describes the action that triggers the behavior being tested. This is typically a single action: a user clicks a button, submits a form, or makes an API call.

Then states what should happen as a result of the action. This describes the observable outcomes that verify correct behavior.

Here is a simple example:

Scenario: Successful login with valid credentials
  Given the user is on the login page
  And the user has an active account with email "user@example.com"
  When the user enters email "user@example.com" and password "validpass123"
  And the user clicks the login button
  Then the user should see the dashboard
  And the user should see a welcome message

Writing Effective Scenarios

Good scenarios are declarative, not imperative. They describe what should happen, not how to accomplish it.

Poor scenario (imperative):

Scenario: User login
  Given I am on "https://example.com/login"
  When I fill in the field with id "email" with "user@example.com"
  And I fill in the field with id "password" with "pass123"
  And I click the element with class "submit-btn"
  Then I should see text "Welcome" in element with id "greeting"

Better scenario (declarative):

Scenario: User login
  Given a registered user with email "user@example.com"
  When the user logs in with valid credentials
  Then the user sees the welcome dashboard

The declarative version focuses on business behavior. Implementation details stay in the step definitions. If the UI changes, you update step definitions without touching scenarios.

And and But Keywords

Use And to add additional conditions within the same section. Use But to express negative conditions.

Scenario: Apply discount to eligible order
  Given a customer has items in their cart
  And the customer has a valid discount code "SAVE20"
  And the order total exceeds $50
  When the customer applies the discount code at checkout
  Then the order total should decrease by 20%
  But the discount should not apply to shipping costs

Gherkin Syntax Fundamentals

Gherkin is the structured language for writing BDD specifications. Files use the .feature extension and follow a specific format.

Feature and Scenario Structure

Every feature file starts with a Feature keyword followed by a description:

Feature: Shopping Cart Management
  As an online shopper
  I want to manage items in my shopping cart
  So that I can purchase products I need
 
  Scenario: Add item to empty cart
    Given the shopping cart is empty
    When the customer adds a product with SKU "LAPTOP-001" to the cart
    Then the cart should contain 1 item
    And the cart total should reflect the product price
 
  Scenario: Remove item from cart
    Given the shopping cart contains product "LAPTOP-001"
    When the customer removes the product from the cart
    Then the cart should be empty

The Feature description often follows the user story format: As a [role], I want [feature], So that [benefit]. This is not required but provides context about why the feature exists.

Scenario Outlines for Data-Driven Tests

When testing the same behavior with different data, use Scenario Outline with Examples tables:

Scenario Outline: Password validation rules
  Given a user is creating an account
  When the user enters password "<password>"
  Then the system should display "<validation_result>"
 
  Examples:
    | password        | validation_result              |
    | abc             | Password too short             |
    | abcdefgh        | Password needs a number        |
    | abcdefg1        | Password needs uppercase       |
    | Abcdefg1        | Password accepted              |
    | Abcdefg1!       | Password accepted              |

The test runs once for each row in the Examples table. Placeholders like <password> get replaced with actual values from the table.

Background for Common Setup

When multiple scenarios share setup steps, use Background to avoid repetition:

Feature: Order Management
 
  Background:
    Given a logged-in customer
    And the customer has an active order
 
  Scenario: View order details
    When the customer views their order
    Then they should see the order items and total
 
  Scenario: Cancel order
    When the customer cancels their order
    Then the order status should change to "cancelled"
    And the customer should receive a cancellation email

Background runs before each scenario in the feature file. Keep Background steps minimal and directly relevant to all scenarios.

Tags for Organization

Tags help organize and filter scenarios:

@checkout @payment
Feature: Payment Processing
 
  @smoke
  Scenario: Successful credit card payment
    Given a valid credit card
    When the customer completes checkout
    Then the payment should be processed
 
  @slow @integration
  Scenario: Payment gateway timeout recovery
    Given a slow payment gateway response
    When the customer completes checkout
    And the gateway times out
    Then the system should retry the payment

Tags enable running specific subsets of tests. Common uses include marking smoke tests, slow tests, work-in-progress scenarios, or tests requiring specific environments.

BDD Tools and Frameworks

Cucumber

Cucumber is the most widely used BDD framework. It supports multiple programming languages through different implementations:

• Cucumber-JVM for Java and Kotlin
• Cucumber.js for JavaScript and TypeScript
• Cucumber-Ruby for Ruby
• Cucumber-Go for Go

Cucumber parses Gherkin feature files and matches steps to step definitions written in code:

// Step definitions in Java
public class LoginSteps {
 
    @Given("a registered user with email {string}")
    public void aRegisteredUserWithEmail(String email) {
        testUser = userService.createUser(email);
    }
 
    @When("the user logs in with valid credentials")
    public void theUserLogsInWithValidCredentials() {
        loginPage.login(testUser.getEmail(), testUser.getPassword());
    }
 
    @Then("the user sees the welcome dashboard")
    public void theUserSeesTheWelcomeDashboard() {
        assertTrue(dashboardPage.isDisplayed());
        assertTrue(dashboardPage.hasWelcomeMessage());
    }
}

SpecFlow

SpecFlow brings BDD to the .NET ecosystem. It integrates with Visual Studio and supports C# step definitions:

[Binding]
public class LoginSteps
{
    [Given(@"a registered user with email ""(.*)""")]
    public void GivenARegisteredUserWithEmail(string email)
    {
        _testUser = _userService.CreateUser(email);
    }
 
    [When(@"the user logs in with valid credentials")]
    public void WhenTheUserLogsInWithValidCredentials()
    {
        _loginPage.Login(_testUser.Email, _testUser.Password);
    }
 
    [Then(@"the user sees the welcome dashboard")]
    public void ThenTheUserSeesTheWelcomeDashboard()
    {
        Assert.IsTrue(_dashboardPage.IsDisplayed());
    }
}

SpecFlow integrates with popular .NET test frameworks including NUnit, xUnit, and MSTest.

Behave (Python)

Behave provides BDD capabilities for Python projects:

# features/steps/login_steps.py
from behave import given, when, then
 
@given('a registered user with email "{email}"')
def step_registered_user(context, email):
    context.test_user = create_user(email)
 
@when('the user logs in with valid credentials')
def step_user_logs_in(context):
    context.login_page.login(context.test_user.email, context.test_user.password)
 
@then('the user sees the welcome dashboard')
def step_sees_dashboard(context):
    assert context.dashboard_page.is_displayed()

Other BDD Frameworks

• Behat for PHP applications
• Gauge from ThoughtWorks, supports multiple languages with markdown-style specifications
• JBehave Java framework that preceded Cucumber
• Concordion uses HTML specifications instead of Gherkin

Choosing a Framework

Select based on your technology stack and team needs:

Implementing BDD in Your Team

Starting with Discovery

Before writing any Gherkin, conduct discovery sessions. Gather business stakeholders, developers, and testers. Discuss upcoming features using concrete examples.

Ask questions like:

• What is the simplest example of this working correctly?
• What happens when something goes wrong?
• Are there edge cases we should handle?
• What existing behavior might this affect?

Document examples on index cards or a whiteboard. Do not write Gherkin during discovery. Focus on understanding.

From Examples to Scenarios

After discovery, convert examples into Gherkin scenarios. One person typically writes the first draft. Others review for clarity and completeness.

Keep scenarios at the right level of abstraction. They should describe business behavior, not implementation details or UI mechanics.

Connecting Scenarios to Code

Step definitions connect Gherkin to actual test code. Keep step definitions thin. They should call into a test automation layer, not contain complex logic themselves.

// Thin step definition
@When("the customer places an order")
public void theCustomerPlacesAnOrder() {
    checkoutPage.placeOrder();
}
 
// Automation layer handles complexity
public class CheckoutPage {
    public void placeOrder() {
        fillShippingAddress(testContext.getShippingAddress());
        selectPaymentMethod(testContext.getPaymentMethod());
        confirmOrder();
        waitForOrderConfirmation();
    }
}

This separation makes scenarios stable when UI changes. Only the automation layer needs updating.

Maintaining Living Documentation

BDD scenarios form living documentation that stays current with code. For this to work:

• Run scenarios in CI/CD pipelines
• Fix failing scenarios immediately
• Update scenarios when requirements change
• Delete scenarios for removed features
• Review scenarios during code reviews

Scenarios lose value when they fail intermittently or become disconnected from actual application behavior. A passing scenario that does not test what it claims is worse than no scenario at all.

BDD vs TDD: Key Differences

BDD evolved from TDD, but they serve different purposes:

TDD and BDD complement each other. TDD drives low-level design decisions. BDD captures high-level behavior requirements. Many teams use both:

1. Three Amigos discuss feature and write BDD scenarios
2. Developer starts implementing
3. TDD guides internal code design
4. BDD scenarios verify feature works correctly

When to Use BDD

BDD works well when:

Complex domain logic makes requirements difficult to communicate. BDD scenarios provide concrete examples that reduce ambiguity.

Multiple stakeholders need to agree on behavior. Gherkin serves as a common language between business and technical teams.

Living documentation is valuable. Teams maintaining long-lived products benefit from specifications that stay current.

Acceptance criteria need precision. BDD scenarios clarify what "done" means for each feature.

Regulatory compliance requires traceability. BDD creates auditable links between requirements and tests.

BDD may not be worth the overhead when:

• Building simple CRUD applications with obvious behavior
• Working alone or in very small teams with constant communication
• Prototyping or exploring where requirements change constantly
• Teams lack commitment to maintain scenarios over time

BDD at Different Test Levels

BDD is commonly used for acceptance testing and integration testing. Using BDD for unit tests is usually overkill. The overhead of Gherkin does not pay off for fine-grained code behavior.

BDD Best Practices

Write Scenarios Before Code

Scenarios should exist before implementation begins. This ensures scenarios describe intended behavior rather than documenting existing code. Writing scenarios after code often produces scenarios that test implementation rather than requirements.

Keep Scenarios Independent

Each scenario should run independently without depending on other scenarios. Avoid scenarios that must run in sequence. Use Background for common setup instead of relying on state from previous scenarios.

Use Domain Language

Scenarios should use the same terms your business uses. If business people say "customer," do not write "user." If they say "order," do not write "purchase transaction." Consistent terminology reduces translation errors.

Avoid Incidental Details

Include only details essential to the behavior being tested:

Too many details:

Scenario: Apply member discount
  Given a customer named "John Smith" with email "john@example.com"
  And the customer registered on "2024-01-15"
  And the customer is a premium member
  And the customer has a shipping address at "123 Main St"
  When the customer checks out with item "Blue T-Shirt" priced at $25.00
  Then the total should be $21.25

Focused on essential behavior:

Scenario: Premium members receive 15% discount
  Given a premium member customer
  When the customer purchases an item priced at $25.00
  Then the total should be $21.25

One Scenario, One Behavior

Each scenario should test one specific behavior. If a scenario has many Then statements testing different things, split it into multiple scenarios.

Make Step Definitions Reusable

Write step definitions that can be reused across scenarios. Use parameterization to handle variations:

@Given("{word} is logged in")
public void userIsLoggedIn(String userType) {
    User user = createUser(userType);
    loginAs(user);
}
 
// Matches: "admin is logged in", "customer is logged in", etc.

Common BDD Mistakes to Avoid

Writing Scenarios After Code

When scenarios document existing code instead of specifying intended behavior, they become change detectors rather than specifications. The value of BDD comes from conversations before development.

Too Many Scenarios

Every scenario requires maintenance. Teams sometimes write scenarios for trivial behavior that does not need specification. Focus on scenarios that clarify ambiguous requirements or document important business rules.

Imperative Steps

Steps describing UI interactions break when interfaces change. Write declarative steps describing what happens, not how:

# Avoid
When I click the button with id "submit-order"
 
# Prefer
When I place the order

Ignoring Failing Scenarios

Teams sometimes mark scenarios as "pending" or ignore failures. Failing scenarios indicate either broken code or outdated specifications. Both need immediate attention.

Skipping the Conversation

Writing Gherkin without Three Amigos discussions defeats the purpose. The specifications are valuable, but the shared understanding from conversations is more valuable.

Treating BDD as a Testing Tool

BDD is a collaboration method first. Automated tests are a useful byproduct. Teams focused only on test automation miss the communication benefits that make BDD valuable.

Conclusion

Behavior-Driven Development bridges communication gaps between business and technical teams through executable specifications written in plain language. The Given-When-Then format provides structure while remaining accessible to non-technical stakeholders.

Success with BDD requires commitment to collaboration. Three Amigos discussions before development matter more than perfect Gherkin syntax. Scenarios written after code provide less value than scenarios that drive development.

Tools like Cucumber and SpecFlow make implementing BDD straightforward across technology stacks. Choose a framework that fits your language ecosystem and team capabilities.

BDD works best for complex domains where requirements need clarification, multiple stakeholders must agree on behavior, and living documentation provides ongoing value. For simpler applications or teams without stakeholder involvement, the overhead may not justify the benefits.

Start small. Pick one feature for your next sprint. Conduct a Three Amigos session. Write scenarios before development. Automate those scenarios. Evaluate whether the improved communication and documentation justify continuing with BDD on future features.

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