The general question considered in this review is what is the role of actin polymerization in cell motility? Two specific questions are (a) do individual actin filaments undergo net assembly or disassembly as part of a process in cell motility, and (b) does the energy of assembly or disassembly drive processes in cell motility? Actin filaments have anisotropic distributions in cells, which often have asymmetric shapes. During development, structures with high concentrations of actin filaments appear, and during normal physiologic activity of many non-muscle cells, the actin distribution changes. Does this anisotropy occur because filaments move about the cytoplasm, or because subunits move, with net filament assembly at certain places, and with net disassembly at other places? Changes in intracellular actin distribution often accompany cell movement or changes in cell shape. Does the energy available from actin polymerization drive the motions of the cell? Interest in this question has increased because mutant cells lacking myosin are normal in some aspects of cell motility. These mutants and other investigations into myosin funtion have been reviewed recently (59, 60, 95) (see Pollard, Doberstein, Zot this volume). This review is not complete in its description of the literature and concentrates on a few systems and experimental approaches. Recent reviews on related topics include references 71, 77, 82, and 96.