The Model-Driven Development (MDD) describes and maintains models of the system under development. The Unified Modeling Language (UML) supports a set of semantics and notation that addresses all scales of architectural complexity by using a MDD perspective. Use Cases and Activity Charts are two modeling techniques of the UML. The first one helps the designers to identify the requirements of the system discovering its high level functionality. The second one helps them to specify the internal behaviour of a certain entity or subsystem of the software developed, such as a database, a graphical interface, a software component, or any specific software. However, there is not a direct way to relate/model the requirements (use cases) with their internal behavior (activity charts). In this paper we present a method for describing use cases with activity charts. Our technique also allow us to identify the two main use case relationships ---include and generalization--- by means of activity charts. As a case study, we will show how to use the activity charts to describe graphical user interfaces (GUI) from use cases. In particular, we will show an Internet book shopping system example.