Cardiac and skeletal muscle contraction are activated by Ca2+ binding to specific regulatory sites on the striated muscle thin filament. The thin filament is a large allosteric assembly, containing multiple copies of actin, tropomyosin, and the three troponin subunits (troponin C, troponin I, and troponin T). This review describes recent developments in understanding the structure and dynamics of these proteins and how they function to regulate contraction. Emphasis is placed on the cardiac thin filament and on the features that distinguish it from the skeletal muscle system. The discussion also emphasizes current knowledge of the protein-protein interactions involved in the cooperative processes of thin filament assembly and regulation.