Semi-presidentialism - where the constitution provides for both a directly elected fixed-term president and a prime minister and cabinet collectively responsible to the legislature - is an increasingly common form of government. For many observers cohabitation is the Achilles heel of semi-presidentialism. This article aims to identify the conditions that are associated with the onset of cohabitation. We specify a number of hypotheses that predict the conditions under which cohabitation should occur. We then test our hypotheses on the basis of a new data set that records every case of cohabitation in all semi-presidential electoral democracies from 1989 to 2008 inclusive. We confirm that cohabitation is more likely to occur in countries with a premier-presidential form of semi-presidentialism and show that it is more likely to follow an election that occurs midway through a parliamentary or presidential term, and that when cohabitation follows a presidential election, it is likely to do so in a country where there is only a very weak president. Overall, we find that the conditions under which cohabitation is most likely to occur are also the ones where it is most easily managed. Thus, our findings imply that cohabitation is not likely to be as problematic as the existing literature would suggest. © 2011 The Authors. Political Studies © 2011 Political Studies Association.