Recent progress in actin structural studies may give us new insight into the role of actin in muscle and cell motility. The crystallographic structure of complexes of actin and profilin (Carlsson et aI., I976; C. Schutt, personal communication) and actin and DNAase I (Suck et al., 198I; Sakabe et al., 1983) is being solved by three different groups, and we may soon have an atomic model for G-actin. Lower resolution electron microscopic work, which will be discussed in this paper, has laid the basis for orienting the actin monomer in the filament. Spectroscopy, X-ray diffraction and electron microscopy are yielding clues about the dynamics of the filament. All of these areas of research will have to be synthesized into a molecular model for the actin filament which will describe the nature of actin's interaction with a large class of other proteins. It is important at the outset to clarify the intended scope of this review. Very little attempt will be made to integrate the wealth of current data from biochemistry, sequencing and spectroscopy into a coherent model for the actin filament. Rather, I will attempt to summarize our present understanding of the low-resolution structure of the filament, the position of tropomyosin on this filament and some aspects of the dynamics of the filament. This, of course, leaves many questions untouched, including the role of troponin.