In this review, we address current concepts and recent experimental results that relate to heterogeneity in the physiologic, biochemical, and morphologic properties of mammalian skeletal muscle fibers. Recent data reinforce some of these current concepts while questioning others. The biochemical bases of the concepts of fiber types continue to evolve, particularly in reference to the combinations of the myosin light and heavy chains that are expressed in most mammalian skeletal muscle fibers. Further, it is becoming increasingly clear that specific myosin isoforms relate to contractile function. What remains unclear, however, are the bases for a continuum in functional properties, eg, the maximum velocity of shortening, rather than distinct increments in these properties as might be predicted based on myosin isoforms alone. Clearly, there are factors other than myosin that play a role in defining the characteristics of a fiber, and some of these other factors are discussed. A second general issue addressed in this review is the source of the control of the heterogeneity among fibers of a skeletal muscle. Data are presented that demonstrate that this heterogeneity is not simply a function of the patterns of activation of motoneurons. It appears that the motoneuron can maintain practically all type-related features of fibers in the absence of neuromuscular activity. Although the motoneuron can influence muscle protein expression, it is equally clear that there are myogenic and hormonal factors that have significant regulatory roles. In effect, muscle fiber types reflect a complex interaction of multiple sources of control of protein expression, and the net effect of the control ultimately defines its functional properties. Current Opinion in Rheumatology 1993, 5:695-705