This volume intends to provide a comprehensive overview on the mecha nisms of muscle contraction and non-muscle cell motility at the molecu lar and cellular level, not only for investigators in these fields but also for general readers interested in these topics. A most attractive feature of various living organisms in the animal and plant kingdoms is their ability to move. In spite of a great diversity in the structure and function of various motile systems, it has frequently been assumed since the nineteenth century that all kinds of motility are essentially the same. Based on this assumption, some investigators in the nineteenth century thought that the mechanisms of motility could better be studied on primitive non-muscle motile systems such as amoeboid movement, rath er than on highly specialized muscle cells. Contrary to their expectation, however, the basic mechanisms of motility have been revealed solely by investigations on vertebrate skeletal muscles, since a monumental discovery of Szent-Gyorgyi and his coworkers in the early 1940s that muscle contraction results from the interaction between two different contractile proteins, actin and myosin, coupled with ATP hydrolysis.