Contractile proteins are present as multiple isoforms encoded by multigene families. The numbers of known isoforms of the myosins, actins, tropomyosins, and troponins are summarized, and the mechanisms by which this diversity has been generated are discussed. Two major strategies can be distinguished, one that depends on gene duplication events leading to the appearance of complete new structural genes, and a second that involves differential splicing of exons within a single complex gene. This latter mechanism is particularly prevalent among the contractile protein families. The developmental regulation of isoform expression is reviewed in the context of these strategies. The biological significance of contractile protein polymorphism is discussed.