Implementing, validating, modifying, or reengineering an object-oriented system requires an understanding of the object and class interactions which occur as a program executes. This work seeks to identify, visualize, and analyze interactions in object-oriented program executions as a means for examining and understanding dynamic behavior. We have discovered recurring interaction scenarios in program executions that can be used as abstractions in the understanding process, and have developed a means for identifying these interaction patterns. Our visualizations focus on supporting design recovery, validation, and reengineering tasks, and can be applied to both object-oriented and procedural programs.