It is the good reader that makes the good book. RALPH WALDO EMERSON, Society & Solitude. In the course of two projects, the author of this book was involved in the design of the platforms PARFORM [CS93) and LOLA [Cap94), [CS) for the support of parallel computing in distributed systems. The former system was geared towards the highly efficient use of idle resources in networks of workstations, and the latter system was intended as a scalability study: How many workstations in the global Internet can be used simultaneously for solving a massively parallel problem? In one of the experiments conducted with these systems, up to 800 workstations on all five continents were cooperating for the solution of a search problem from molecular biology [Cap94). The most important lessons which the author was forced to learn during the course of these projects were not to rely on any documentation of network-and low-level system-calls, to use neither common sense nor mathematical logic during the design of a large distributed system, but to be happy with a working program, and not to ask, why it would work.