Why do coalition parties break policy commitments? Before coalition governments take over executive offices, they typically engage in intensive coalition negotiations and publish a comprehensive coalition agreement in which they provide a detailed account of the policy reforms they plan to enact in government. However, previous case study evidence shows that only about two thirds of all the policy reforms promised in coalition agreements were actually enacted. Despite its importance, the question of how coalition governments translate policy promises into policy has received little academic attention to date. In order to address this gap, we develop a new theoretical argument on policy implementation in coalitions. More specifically, we argue that the decision not to enact policy pledges can be explained by the preference configuration within the cabinet and by changes in public opinion. To test our argument, we have compiled a new comparative dataset on the enactment of more than 12’000 policy pledges by 102 coalition governments in 24 Western and Eastern European countries from 2000 to 2015. Our results have major implications for our understanding of coalition governance, multiparty competition and policy making in multiparty democracies.