Behaviour-Driven Development (BDD) is an agile software development process that encourages collaboration between developers, quality assurance teams, and customers. It involves writing user stories to define desired functionality and writing automated tests to validate stories. BDD tests are written in a language that business stakeholders can understand. It works in an "outside-in" approach where tests are written before code to ensure business needs are met. BDD aims to deliver working, tested software frequently in short iterations to provide business value.