Recent years have seen the proliferation of new computer designs that employ parallel processing in one form or another in order to achieve maximum performance. Although the idea of improving the performance of computing machines by carrying out parts of the computation concurrently is not new (indeed, the concept was known to Babbage ), such machines have, until fairly recently, been confined to a few specialist research laboratories. Nowadays, parallel computers are commercially available and they are finding a wide range of applications in chemical calculations. The purpose of this volume is to review the impact that the advent of concurrent computation is already having, and is likely to have in the future, on chemical calculations. Although the potential of concurrent computation is still far from its full realization, it is already clear that it may turn out to be second in importance only to the introduction of the electronic digital computer itself.