The in vivo migration of eukaryotic cells over surfaces is important to a wide range of physiological processes, such as the development of the embryo, defense against infections, wound healing, the homing of lym­ phocytes to lymphoid organs, and tumor metastases (cf Trinkaus 1 984). Despite much study, the directed migration of cells is not yet well under­ stood, no doubt because it is indeed a complex problem. Part of the complexity is due to the great variety of locomotory phenotypes that are expressed by different types of motile cells. Many different cells can migrate under appropriate circumstances: In this article we refer to the directed migration of fibroblasts, endothelial cells, epithelial cells, macrophages, granulocytes, fish epidermal keratocytes, and amebae. Many of these cell types show distinct locomotory characteristics. However, most inves­ tigators agree that common mechanisms must operate in all cell migration.