Clusters of personal computers are becoming a viable, low-cost, alternative to traditional high-performance computing platforms as well as to more expensive networks of workstations. This is due in part to the better performance/cost ratio that characterizes personal computer hardware and in part to the outcome of various research projects aimed at producing high-speed communication systems out of off-the-shelf local area network technology. PC clusters are starting to be used for commercial applications as well as in research environments.