Movement is a fundamental property of life. Most forms of movement that we encounter in the living world—be it the transport of a tiny vesicle or the swimming of a whale—have a common molecular basis. They are generated by motor proteins that use the energy derived from the hydrolysis of adenosine triphosphate (ATP) to take nanometer-scale steps along a cellular track. Their actions power the movements we see, involving possibly just a single motor molecule in the case of the vesicle and phenomenally large arrays of billions and billions of motors in the case of the whale. Understanding the molecular basis of the behavior of these molecules is a prerequisite to understanding cellular and organismal motion.