Although there are numerous studies of political parties the political science of parties remains underdeveloped. Even if the recent attempts at redefinition are not without interest, most situate themselves outside the mainstream of political theory and do not bear sufficiently on the problematical aspects of power and of government. To develop a true political science of parties one has to be able to respond to three questions which seem central to a political scientist's concern with parties: who governs in the parties? do the parties really govern? how do the governmental or non-governmental actions of parties affect the society and the support which they obtain there? From these three questions emerge the notions of the leadership power (or internal power), governmental power, and societal power (electoral or non-electoral) of political parties. After a definition of government based on a cybernetic model and a logical definition of power are presented, the notions of leadership power, governmental power, and societal power are successively analysed for the purpose of making them operational. The last part of the article deals with the interdependence of these three powers. It aims at outlining a political theory of parties by drawing together a number of propositions which have hitherto appeared unrelated.