The regulation of microtubule stability by severing of the polymer along its length is a newly appreciated and potentially important mechanism for controlling microtubule function. Microtubule severing occurs in living cells, but direct observation of this event is infrequent. The paucity of direct observations leave open to question the significance of regulated microtubule severing in the control of microtubule organization. Nevertheless, several lines of evidence suggest that microtubule severing is an important cellular activity. First, the ATP-dependent microtubule-severing activity of katanin is well documented. Katanin is found in most cell types and is enriched at MTOCs. Although it is possible that katanin does not sever microtubules in vivo, this seems unlikely. Second, a physiological event, deflagellation, has been shown to depend on microtubule severing. The deflagellation system of Chlamydomonas has provided a genetic approach to the problem of microtubule severing. The FA genes are essential for the regulated severing of axonemal microtubules during deflagellation, but whether these genes define new severing proteins or whether they are important for katanin activity remains to be determined. Microtubule severing is a relatively new area of investigation and there are still many more questions than answers. It is anticipated that the recent cloning of katanin and the introduction of a genetic model system will soon lead to significant breakthroughs in this problem.