Barriers to mobility, particularly those imposed by state-level licensure procedures have received growing attention in the media and in reform discussions. Much of this attention is driven by shortages of teachers in some regions and subject areas and the fact that barriers to mobility make it more difficult for states to address such shortages by drawing on whatever surplus of labor may exist out of state. But there are reasons to expect that lowering barriers to mobility would be beneficial to the teacher labor market even in the absence of shortages. To the extent that teachers value locational flexibility, lowering barriers to mobility could make the teaching profession more desirable to prospective entrants and reduce attrition among experienced teachers who may opt out of the profession following a move to a new state. The authors describe how licensure procedures and pension systems impose costs on teachers who cross state borders, discuss the evidence on whether teachers are responding to these disincentives to mobility, and consider possible reforms that would lower barriers to teacher mobility.