Microtubules (MTs) and actin filaments have an intrinsic molecular polarity based on the unipolar stacking of their asymmetrical subunits. Filament polarity is believed to be important for many cellular activities. For example, the interaction of actin and myosin that produces shortening of the sarcomere in striated muscle depends on the correct vectorial orientation of actin filaments relative to myosin. While a convenient polarity marker for actin has been available for over 20 years, similar biochemical tools became available for MTs only a few years ago. The purpose of this paper is to summarize some observations on the relation between MT polarity and MT involvement in cellular motility. For more complete accounts of MT biochemistry and physiology, the reader is referred to recent overviews and monographs (Roberts and Hyams, 1979; DeBrabander and DeMey, 1980; Gunning and Hardham, 1982; Sakai et al., 1982; Dustin, 1984; McKeithan and Rosenbaum, 1984).