Despite extensive research on government and ministerial duration, there are still relatively few comparative studies of ministerial survival and accountability. This thesis uses the analytical tools provided by the agency theory to study the variation in the length of ministerial tenure and makes an original contribution to the study of this topic from two points of view. First, whereas earlier literature studied cabinet members solely as prime ministerial agents, this thesis explicitly adopts a three-principal-agent model to analyse the accountability of cabinet ministers to their own parties, directly-elected presidents, and prime ministers. For this reason, the variation in the length of ministerial tenure is analysed in countries with semi-presidential constitutions, which maximise the intra- and inter-case variation in principal-agent relationships. Adopting an interactionist view on the factors that explain the variation in the ability of presidents, prime ministers, and parties to control cabinet members in semi-presidential systems, we expect that the ministers’ survival in office depends on the interaction between institutional scenarios and party relationships between the minister, the president and the prime minister. The second contribution of this thesis is empirical. A unique data set on the tenures of French, Portuguese and Romanian ministers during two legislative terms has been collected in order to test these theoretical expectations. In addition to fixed characteristics at the moment of appointment, the data set records resignation calls and conflicts between ministers and their principals. This data allow one to measure the variation in the political influence of presidents, prime ministers and political parties that is not easily observable. The results indicate that principals who act as de facto party leaders are more likely to control the process of ministerial deselection under certain institutional scenarios. Such a study is important because it addresses the link between institutional design and political accountability and emphasises the extra- constitutional factors accounting for the variation in political practices across and within similar institutional frameworks over time.