To investigate the program-execution data efficiently, we must be able to view the data at different levels of detail. In our visualization approach, we represent software systems at three different levels: statement level, file level, and system level. At the statement level, we represent the actual code. The representation at the file level provides a miniaturized view of the source code similar to the one used in the SeeSoft system (Eick et al., 1992). The system level uses treemaps (Shneiderman, 1992 and Bruls et al., 2000) to represent the software and is the most abstracted level in our visualization. At each level, coloring is used to represent one- or two-dimensional information about the code, using the colors’ hue and brightness components. The coloring technique that we apply is a generalization of the coloring technique defined for fault-localization by Jones and colleagues (2001). GAMMA℡LA is a toolset that implements our visualization approach and provides capabilities for instrumenting the code, collecting program-execution data from the field, and storing and retrieving the data locally. GAMMA℡LA is written in Java, supports the monitoring of Java programs, and consists of three main components: an instrumentation, execution, and coverage tool, a data collection daemon, and a program visualizer.