The abdity to count events in a program’s execution M reqmred by many program analysls applications. We present an instrumentation method for efficiently counting events in a program’s execution, with support for on-line queries of the event count, Event counting differs from basic block profiling in that an aggregate count of events is kept rather than a set of counters, Due to this difference, solutions to basic block profiling are not well suited to event counting. Our algorithm finds a subset of points in a program to instrument, while guaranteeing that accurate event counts can be obtained efficiently at every point m the execution.