A heterogeneous multicomputer is a multicomputer composed of two or more different types of processors. This paper describes the rationale for heterogeneity in a multicomputer and gives a typical example of a heterogeneous system in the form of a RACE multicomputer composed of a mixture of Analog Devices' SHARC 21060 and the IBM/Motorola/Apple PowerPC 603p processors. These two processors have complementary attributes, and the advantages and limitations of each are described. Multicomputers generally implement a sequence of different processing algorithms. The "optimal" processor that maximizes throughput at each step in the processing flow is generally a function of the algorithm to be executed at that step. Other factors that also influence the optimal mix of processors in a heterogeneous multicomputer include physical processing density, hardware cost, and ease of programmability.