The dominant structure of knowledge in the twentieth century was division into domains of disciplinary specialization. In the latter half of the century this system was challenged by an increasing number of interdisciplinary activities. This chapter examines typologies of interdisciplinary activities, identifying patterns of consensus and fault lines of debate from the first major classification scheme in 1970 and continues to recent taxonomies that recognize new developments. The chapter compares similarities and differences in a framework of multidisciplinary juxtaposition and alignment of disciplines, interdisciplinary integration and collaboration, and transdisciplinary synthesis and trans-sector problem solving. It further distinguishes major variants of methodological versus theoretical interdisciplinarity, bridge building versus restructuring, and instrumental versus critical interdisciplinarity. Typologies are neither neutral nor static. They reflect choices of representation in a semantic web of differing purposes, contexts, organizational structures, and epistemological frameworks. They reassert, extend, interrogate, and reformulate existing classifications to address both ongoing and unmet needs.