There is no paucity of definitions of the concept of ‘institution’, and often they are somewhat over-encompassing. The chapter discusses several approaches and elaborates on the implications of each of them. The conclusion is that what constitutes an institution is not generalisable and depends largely on the researcher, the object of research and the research question. Different conceptions of institution serve egregiously different research programmes and different types of situations. However, too often different approaches extend their specific understandings of ‘institutions’ to the whole world of institutions, presenting themselves as general theories of institution. This generates inconsistencies when the approach and its theoretical results are applied outside the original scope and extended to the entire universe of institutions. Discussion of the different approaches helps to identify the core properties each highlights and to reconstruct the semantic field of the concept of institution. I focus on the main characterising elements of different understandings.