In this paper we will show how to obtain a better comprehension of the behavior of large and complex software systems using visualization and program animation. Use cases are used in the software development industry for the purpose of better understanding of software systems. The problem is that use cases for large and complex software systems are themselves large and complex. We have built a tool called Scenario Explorer to help the user handle this complexity by guiding him or her through the use cases. The user is provided with just enough information to comprehend the behavior of interest. Tools like this are crucial for developers that need to obtain a better comprehension of a whole software system, for example newcomers in a project or managers. Scenario Explorer was built at Ericsson and applied to a subset of a large and complex software system.