This chapter explains how to detail use cases as text and how to evolve them to system sequence diagrams in order to discover which high-level operations should be implemented by the system. It concentrates on explaining what is really necessary for a use case description during Elaboration, when its functionality must be completely understood. Technical and interface details are still avoided. Only information flows, including variant and exception flows, are included in an essential use case description; this chapter explains how to discover them systematically. It also explains how to transform a textual and informal description of a use case into a sequence diagram, which is a higher ceremony artifact.