Object-oriented software is designed by introducing classes and their relationships. This design is then transformed into source code by a software developer. But when the software is executed, the developer loses sight of the classes he created during the design and coding of the software.This thesis describes the design and implementation of a tool which shows the developer which classes interact during execution. This tool, called TraceVis, allows the developer to study the executions of Java programs. It uses bytecode instrumentation techniques to extract relevant information from these executions. This information is presented using several visualizations.To determine the usefulness of TraceVis it was evaluated at the end of the project, by comparing it to similar tools, and by running it through a few test cases.