The human genome contains about 3 x 10(9) base pairs which, based on average estimates for the size of coding regions, might suggest an upper limit of about 3 million genes. Evidence from a variety of approaches indicates a much lower figure--in the range 40,000-100,000 genes. Renaturation kinetic analysis reveals the presence of many repetitive elements in the human genome. Some of these are quite simple and highly reiterated, others are more complex and exist in fewer copies. The distribution and function of some of these families and the evolution of complex genomes from simpler ones are discussed.