The ch-TOG/XMAP215 family of proteins bind directly to microtubules and appear to play an essential role in stabilizing spindle microtubules. These proteins stabilize microtubules mainly by influencing microtubule plus-end dynamics, yet, in vivo, they are all strongly concentrated at spindle poles, where the minus ends of the microtubules are concentrated. In Drosophila embryos, the centrosomal protein D-TACC is required to efficiently recruit ch-TOG/Msps to centrosomes. In humans, ch-TOG and the three known TACC proteins have been implicated in cancer, but their functions are unknown. Here we extensively depleted TACC3 and ch-TOG from HeLa cells using RNA interference. In TACC3-depleted cells, spindles are well organized, but microtubules are partially destabilized and ch-TOG is no longer concentrated on spindle microtubules. In ch-TOG-depleted cells, relatively robust spindles form, but the spindles are highly disorganized. Thus, in human somatic cells, ch-TOG appears to play a major role in organizing spindle poles, and a more minor role in stabilizing spindle microtubules that is, at least in part, mediated via an interaction with TACC3.