JUnit is the most important testing framework for the JVM and one of the most influential in software engineering in general. JUnit 5 is the next generation of JUnit, and its first General Availability (GA) version (5.0.0) was released on September 10, 2017. As we will discover, JUnit 5 supposes a small revolution with respect to JUnit 4, providing a completely new architecture, programming, and extension model. This chapter covers the following content:

• Road to JUnit 5: In the first section, we will discover the motivation to create a new major version of JUnit (that is, the limitations of JUnit 4), the design principles guiding the development of JUnit 5, and finally the details of the JUnit 5 open source community.
• JUnit 5 architecture: JUnit 5 is a modular framework composed of three major components, named Platform, Jupiter, and Vintage.
• Running tests in JUnit 5: We will discover how to run JUnit 5 tests using popular build tools, such as Maven or Gradle, and also with IDEs such as IntelliJ or Eclipse.
• The extension model of JUnit 5: The extension model allows for third-party libraries and frameworks to extend the JUnit 5 programming model with their own additions.