To understand international cooperation and discord, it is necessary to develop a knowledge of how international institutions work, and how they change. The assumption of substantive rationality has proved a valuable tool in pursuing such knowledge. Recently, the intellectual predominance of the rationalistic approach has been challenged by a “reflective” approach, which stresses the impact of human subjectivity and the embeddedness of contemporary international institutions in pre-existing practices. Confronting these approaches with one another helps to clarify the strengths and weaknesses of each. Advocates of the reflective approach make telling points about rationalistic theory, but have so far failed to develop a coherent research program of their own. A critical comparison of rationalistic and reflective views suggests hypotheses and directions for the development of better-formulated rationalist and reflective research programs, which could form the basis for historically and theoretically grounded empirical research, and perhaps even for an eventual synthesis of the two perspectives.