-1 The TOP500 [4] provides an opportunity to rank the top supercomputer sites worldwide. In the TOP500 report the 500 highest performing supercomputers are listed. The measurement of performance is based on the LINPACK benchmark as reported in [3]. All the information here is based on the TOP500 of November 1993. We use results from this list because it provides the most comprehensive list of supercomputer sites. Also real LINPACK figures are available for all the machines on the list. Some of the limitations of using LINPACK and relying on the TOP500 are discussed in chapter 
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An alternative ``List of the World's Most Powerful Computing Sites'' is compiled by Ahrendt [1]. This list is based on the NAS Parallel Benchmarks code BT [2]. There is considerable overlap between these two lists. BT as benchmark generally does not perform as well on highly parallel machines as LINPACK, when compared to vector machines. Also BT results are not always immediately available on new machines, so [1] resorts sometimes to estimates and extrapolations. On the other hand the list [1] is updated more frequently. In general, however, the results and the ranking are fairly consistent. The main point here is that we will give some additional information about the supercomputer sites, beyond just a listing of machines. This provides a general overview and pointers to further reading, on what the most powerful supercomputers on earth are actually used for.