The term cytoskeleton in muscle and nonmuscle cells refers to fibrous proteins that are directly or indirectly associated with cell shape, contractility, and/or motility. These cytoskeletal elements include microtubules and associated proteins, microfilaments and their binding proteins, and lastly, intermediate filaments. Even though the structure and function of myosin and tro- pomyosin have been extensively investigated in muscle ceIls, considerably less is known about their functions in nonmuscle ceIls; they are often referred to generically as actin-binding proteins, as are a-actinin, filamin, vinculin, and others. Indeed, most nonmuscle researchers are unaware that the term intermedi- ate filament was derived from the observation that the diameter of intermedi- ate filaments is intermediate between those of actin (5 nm) and myosin (15 nm) and not intermediate between those of actin (5 nm) and microtubules (25 nm). It has therefore been the purpose of this series to attempt to bridge the communication gap that appears to exist between muscle and nonmuscle researchers. For those students and active scientists who are faced with the task of absorbing new data and concepts at an ever-accelerating rate, review essays are an excellent way to keep up. The essays in this volume, as weIl as in the previous ones, focus on topics of current interest. They are intended to be critical rather than exhaustive.