Motor proteins are molecular machines that convert the chemical energy derived from the hydrolysis of ATP into mechanical work used to power cellular motility. In addition to specialized motile cells like muscle fibers and cellular processes like cilia, all eukaryotic cells contain motor proteins (Fig. 1). The reason is that eukaryotic cells are large and their cytosols are crowded with filaments and organelles; as a result, diffusion is too slow to efficiently move material from one part of a cell to another (Luby-Phelps et al. 1987). Instead, the intracellular transport of organelles such as vesicles, mitochondria, and chromosomes is mediated by motor proteins. These proteins include myosins and dyneins that are relatives of the proteins found in the specialized muscle and ciliated cells, as well as members of a third family of motor proteins, the kinesins, which are distantly related to the myosin family.