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The sarcomere, the basic contractile unit of striated muscle cells, is widely accepted as being constructed of two sets of parallel and interdigitated protein filaments that are discontinuous and inextensible. This two-filament sarcomere model provides a structural basis for the powerful sliding-filament theory of muscle contraction. Recent structural, biochemical, and immunocytochemical studies, however, have clearly indicated that the sarcomere has additional filamentous constituents besides thick and thin filament assemblies. The pur pose of this chapter is to outline and develop key evidence that has led to the notion that the sarcomere contains two sets of distinct cytoskeletal filaments that are continuous and extensible. One set of cytoskeletal filaments coexists with thick and thin filaments within the sarcomere (an endosarcomeric lattice); the other set is an extensive network of intermediate filaments enveloping …