The term cytoskeleton has become firmly established in today's scientific vocabulary. Indeed, it is difficult to believe that only ten years ago, it was virtually non-existent. Since then, the modern field of research on the structural organization of the cytoplasm has turned into one of the most productive and rapidly expanding research areas in Cell Biology today. Considerable progress has been made towards the identification of the various structural components of the cytoskeleton and their interactions with one another and with membranes. The first attempts to understand, in molecular terms, complex cellular processes such as shape changes, locomotion, division, and organelle movements have been made. And it is now apparent that the cytoskeleton has impact on other biological processes such as the control of gene expression, protein synthesis, cell cycle regulation, and development. This monograph outlines the basic properties of the major components of the polymeric filament networks and their interactions and associations. Wherever possible, emphasis is placed on more recent references. Any attempt to cover a research field this complex in an introductory mono­ graph is, by necessity, fragmentary, and oversights or omissions are inevitable. I wish to apologize in advance to all those colleagues who feel that their work is not adequately represented.