A distinguishing characteristic of computer science is the enormous gap that exists between the simple instruction sets of our computers and the complexity of useful software. This dichotomy produces intellectual challenges of the highest order. Today many important applications contains several million lines of code, too many for one person to read in a year and impossible for one person to fully understand. VLSI chips are now being manufactured containing more than one hundred million transistors, again posing an almost impossible problem for human comprehension.