The transformations performed by an optimizing compiler have traditionally impeded interactive debugging in source language terms. A prototype system called Navigator has been developed for debugging optimized programs written in Cedar, an Algol-like language. Navigator can be used to monitor program execution flow in the presence of two optimizations: inline procedure expansion and cross-jumping (merging identical tails of code paths that join). This paper describes the problems that these two optimizations create for debugging and Navigator's solutions to these problems. The selected approach collects extra information during the optimization phases of compilation. At runtime, Navigator uses the additional information to hide the effects of the optimizations from the programmer.