Mitosis depends on the mitotic spindle, a subcellular protein machine that uses dynamic microtubules and mitotic motors to assemble itself and to coordinate chromosome movements. Spindle function depends critically on the interplay of microtubule polymer dynamics and the motor proteins and non-motor microtubule-associated proteins (MAPs) that crosslink adjacent microtubules. These microtubule crosslinkers can organize microtubules into bundles with specific polarity patterns and some of them can slide adjacent microtubules in relation to one another. Here, we discuss the functions and mechanisms of action of three such crosslinkers: the motors kinesin-5 and kinesin-14, and the non-motor MAPs of the Ase1p family. © 2009 Elsevier Ltd. All rights reserved.