Categories are not only knowledge-based constructs but used by institutions as vehicles for normative activities that go to shaping social reality and people’s behavior. Moreover, they are used by individuals to categorize their own experiences and orient their conduct. Drawing a line between insiders and outsiders requires defining some criteria of equivalence so as to identify in both concrete and practical terms, the features shared by insiders. Indeed the plurality of concepts and categories of work also permeates academia, with contrasting disciplinary perspectives throwing light on different types and dimensions of work. Categories are developed, taken up and changed within fields of social power. They affect the rights and interests of different groups – empowering some while disadvantaging others – and may contain implicit or explicit hierarchizations. The chapter also presents an overview on the key concepts discussed in this book.