Formal methods can only gain widespread use in industrial software development if they are integrated into software development techniques, tools, and languages used in practice. A symbiosis of software testing and verification techniques is a highly desired goal, but at the current state of the art most available tools are dedicated to just one of the two tasks: verification or testing. We use the KeY verification system (developed by the tutorial presenters) to demonstrate our approach in combining both. 1 What KeY Is KeY is an approach and a system for the deductive verification of object-oriented software. It aims for integrating design, implementation, and quality assurance of software as seamlessly as possible. The intention is to provide a platform that allows close collaboration between conventional and formal software development methods.