Over the past two decades, a growing number of 'outsider parties' have entered governing centre-left and centre-right coalitions across Western Europe. In this introduction, we first define outsider parties as those which - even when their vote-share would have enabled it - have gone through a period of not being 'coalitionable', whether of their own volition or that of other parties in the system. Based on the articles in this issue, we then discuss the problems which outsider parties encounter when entering government and suggest some reasons for the success and failure of these parties in office. Finally, we propose a number of avenues for further research, in particular that of examining differences between the first and subsequent experiences of office for such parties.