It has been thought that motile structures within the cell are driven toward the plus and minus ends of microtubules by the ATPases, kinesin and dynein, respectively. Recently obtained data indicate that this model is far too simplistic. Kinesin is now understood to be one representative of a family of proteins. Another member of the kinesin family has been found to generate force toward the microtubule minus end. Evidence for either a bidirectional dynein, or closely related retrograde and anterograde forms of dynein has also received potent new support. The discovery of a third potential microtubule motor, the GTPase, 'dynamin', complicates matters further.