Actin is a protein that plays an important role in cell structure, cell motility, and the generation of contractile force in both muscle and nonmuscle cells. In many organisms, multiple forms of actin, or isoactins, are found. These are products of different genes and have different, although very similar, amino acid sequences. Furthermore, these isoactins are expressed in a tissue specific fashion that is conserved across species, suggesting that their presence is functionally important and their behavior can be distinguished quantitatively from one another in vitro. In muscle cells, they are differentially distributed within the cell and some are specifically associated with structures such as costameres, mitochondria, and neuromuscular junctions. There is also good evidence for specific isoactin function in microvascular pericytes and in the intestinal brush border. However, the necessity of specific isoactins for various functions has not yet been conclusively demonstrated.