Caldesmon is one of the most abundant proteins detected in smooth muscle and in a number of non-muscle cells. It binds actin, myosin, tropomyosin and calmodulin and is believed to play in smooth muscle a regulatory role similar to that of troponin in striated muscle. Two closely-related isoforms of caldesmon being the products of an alternative splicing of a single gene, have been identified. The high molecular weight (H-CaD) isoform is expressed predominantly in smooth muscle, whereas the low molecular weight (L-CaD) isoform is found in non-muscle tissues and cells. This book examines the anatomy and physiology of caldesmon proteins.