Performance monitoring hardware is available on most modern microprocessors in the form of hardware counters and other registers that record data about processor events. This hardware may be used in counting mode, in which aggregate event counts are accumulated, and/or in sampling mode, in which time-based or event-based sampling is used to collect profiling data. This paper discusses uses of these two modes and considers the accuracy issues raised by each. Implications for the PAPI cross-platform hardware counter interface and the application programmer also are discussed.