Like many other aspects of the behaviour and activities of political parties, the internal organizational procedures and structures of parties have become increasingly subject to external regulation. This chapter explores the empirical and normative dimensions of the legal regulation of the internal operation and functioning of political parties which emerge from a comparative content analysis of party laws and national constitutions of all (33) post-war European democracies. It appears that the internal party organization constitutes an increasingly important feature of party regulation in that many countries, often taking their cue from the classic mass party as a prescriptive model of organization, have introduced specific legal or constitutional requirements on intra-party democracy. We argue that both the explicit and implicit legal regulation of the internal party organization serves as an important indication of the underlying normative preferences of the legislator for internally democratic parties. At the same time, the increased levels of state intervention in internal party politics raises fundamental concerns with democratic legitimacy as well as the state centralization of political participation.