This chapter serves two purposes. On the one hand, it uses the recent debates in International Relations to introduce categories which have become commonplace in the theoretical literature. Although it cannot, of course, cover as much ground as did Hollis and Smith (1990), it seeks to raise the awareness about the significance of meta-theoretical debates. On the other hand, it confronts and discusses the merits and limits of different meta-theoretical typologies, such as the meta-theoretical matrix proposed by Onuf or Hollis and Smith, Keohane's distinction between rationalism and reflectivism, and the postivism versus post-positivism divide.