This study starts with two theoretical puzzles within the rational choice oriented literature on government formation in parliamentary democracies: the relative importance of constitutional rules and the existence of multiple party goals. From these puzzles stem the research questions that guide the study: First, what is the theoretical and empirical link between constitutional arrangements (including rules) and party goals? Second, what are the goals of political parties and how can these be studied? Third, relative to the goals of political parties and other constitutional arrangements, what is the importance of government formation rules for the empirical record of minority and majority governments?Coalition theory provides the theoretical starting point from which the research questions stem. The historical-institutional strand of new institutionalism is used to guide the general understanding of the importance of institutional context. The rational choice oriented strand is used for a detailed study of the design of the Swedish government formation rules and an analysis of how the formation rules affect the goal seeking (micro-logic) of actors.Based on both cross-national data and an in-depth study of Swedish coalition and government formation, the analysis shows that the answer to research question number one is that the link between constitutional arrangements and party goals is one of co-determination. The answer to research question number two is that party leaders pursue four main goals and that this should be an explicit model assumption. The answer to research question number three is that the government formation rules help determine the parties' bargaining positions and for that reason they are of significant importance for the formation of minority and majority governments. / digitalisering@umu