Since the mid-1990s, Green parties have participated in 24 governing coalitions in stable democracies, both from within cabinet and as external support parties in parliament. Despite their similarities, these parties' experiences of coalition have been diverse. This thesis seeks to explain variation among these cases in respect of three outcomes: Green parties' attainment of senior ministerial positions at the moment of government formation  their retention of cabinet office over time  and their electoral outcomes at the end of their spell in coalition. It finds that environmental factors were consistently important for producing these outcomes, but that under many conditions, variation in Green parties' attributes and strategies also played a role. To explain variation in office attainment outcomes, the thesis makes use of an explicitly conjunctural theory that has been developed in the study of support parties. The set of causal factors identified by this theory provides a basis for identifying pathways to high and low office attainment outcomes that are that are empirically consistent and theoretically coherent. In studying office retention outcomes, it develops a framework based on parties' incentives to maximise their electoral and governmental outcomes within a dynamic and institutionally variable setting. It provides a first explanatory account of variation in parties' tenure, identifying a number of pathways to the end of a party's time in office. Finally, the thesis builds on the literature on postincumbency electoral outcomes to identify several paths to post-coalition electoral success and failure. In particular, it suggests that the relatively 'soft' electoral base of Green parties in coalition is an important factor in their losses and that defection from coalition can be electorally beneficial only under restrictive conditions. It identifies a strong tension between office-seeking success and electoral success that presents these parties with especially 'hard choices'.