Motility is a fundamental property of living systems, from the cytoplasmic streaming of unicellular organisms to the most highly differentiated and developed contractile system of higher organisms, striated muscle. Research on var­ ious aspects of motile systems in muscle and undifferentiated or non muscle cells has been developing at an ever more rapid pace in the laboratories of investiga­ tors with a wide variety of backgrounds using methodologies varying from me­ chanics to the most sophisticated physical measurements. Significant contributions to our understanding of motility are coming from the disciplines of cell biology, biochemistry, pharmacology, molecular biology, biophysics, and physiology. The findings have relevance not only to basic scientists but to clinicians in such diverse fields as cardiology and neurology and to scientists in the more traditional physical sciences. Cell and Muscle Motility is a new multivolume series of essays by distinguished research workers in various fields whose work has a common thread of dealing with one aspect or another of motility. The essays are meant to focus on topics of current interest, to be critical rather than exhaustive, and to indicate the current trends of research efforts. The series is intended to foster an interchange of concepts among various workers in the field and to serve as a reference for students and workers who wish to familiarize themselves with the most current progress in motility.