Despite attempts to formalize the semantics of use cases, they remain an informal notation. The informality of use cases is both a blessing and a curse. Whilst it admits an easy learning curve and enables communication between software stakeholders, it is also a barrier to the application of automated methods for test case generation, validation or simulation. This paper presents a precise way of specifying use cases based on a three-level modeling paradigm strongly influenced by UML. The formal syntax and semantics of use case charts are given, along with an example that illustrates how they can be used in practice.