We describe PEPA, the first general on-line performance monitoring tool for Erlang.Erlang is a new non-lazy functional programming language with explicit concurrency. It has been designed especially for building fault tolerant industrial applications that have soft real-time requirements. It has already been used to build substantial software products in the telecommunications area.PEPA gives figures for hardware utilization and processing times as they are required for the development of real-time programs. The tool has a sophisticated graphical user interface that guides the user in a structured search for performance bottlenecks. The on-line mode of operation makes it possible to selectively record data that is actually displayed. This reduces the probe effect introduced by the measurements to the achievable minimum and gives the user the possibility to control the amount of intrusion.