This paper traces the evolution of IBM RISC architecture from its origins in the 1970s at the IBM Thomas J. Watson Research Center to the present-day IBM RISC System/6000® computer. The acronym RISC, for Reduced Instruction-Set Computer, is used in this paper to describe the 801 and subsequent architectures. However, RISC in this context does not strictly imply a reduced number of instructions, but rather a set of primitives carefully chosen to exploit the fastest component of the storage hierarchy and provide instructions that can be generated easily by compilers. We describe how these goals were embodied in the 801 architecture and how they have since evolved on the basis of experience and new technologies. The effect of this evolution is illustrated with the results of several benchmark tests of CPU performance.