Contraction in vertebrate smooth and striated muscles results from the interaction of the actin filaments with crossbridges arising from the myosin filaments. The functions of the actin based thin filaments are (1) interaction with myosin to produce force; (2) regulation of force generation in response to Ca2+ concentration; and (3) transmission of the force to the ends of the cell. The major protein components of smooth muscle thin filaments are actin, tropomyosin and caldesmon, present in molar ratios of 28:4:1 respectively. Other smooth muscle proteins which may be associated with the thin filaments in the cell are filamin, vinculin, alpha-actinin, myosin light chain kinase and calmodulin. We have reviewed the structural and functional properties of these proteins and where possible we have suggested what their function and mechanism of action may be. We propose that actin and tropomyosin are involved in the force producing interaction with myosin, and that this interaction is controlled by a Ca2+-dependent mechanism involving caldesmon, tropomyosin and calmodulin. Vinculin, alpha-actinin and filamin appear to be involved in the attachment of the thin filaments to the cell membrane and their spatial organization within the cell. We conclude that the filaments of smooth muscles share many common properties with those from skeletal muscle, but that they are also quite distinct in terms of both their caldesmon based regulatory mechanism and their mode of organization into a contractile apparatus.