Cellular polarization depends upon the asymmetric transport of components within the cell, often along microtubule paths. Kinesin and cytoplasmic dynein are the most abundant microtubule motors powering the transport of membranous vesicles. Many additional proteins are needed to control motility and coordinate movements with other cellular activities. In this review we will summarize our understanding of the motors, motor associated proteins and their function within cells. We suggest a cyclic model for much of membrane trafficking in which the membrane anchor for the motors, kinectin, plays a central role. In such a model up-regulation of transport results in a faster cycling of kinectin between kinesin and cytoplasmic dynein-dependent transport.