The biology of muscle development has gone full circle--from classical embryonic morphology to cell culture and now back to the embryo at the molecular and cellular levels. Recent advances in modern technology have made possible complex in vitro and in vivo biochemical and ultrastructural analyses of myogenesis. This article correlates theories regarding the events and regulatory factors involved in muscle differentiation and fiber-type formation based on findings from the model system of isolated muscle cells in culture with actual in vivo situations.