 A recurring motif in the literature on levels of organization is to lament the haphazard or unreflective way in which terms such as ‘hierarchy’ or ‘levels’ are applied, and to call for more precise analyses (e.g., Beckner 1974; Bunge 1977; Grene 1987; Korn 2002; Valentine &amp; May 1996). However, surprisingly few philosophers or scientists have taken up the challenge of developing a substantial theory or account of levels of organization. In this section, we discuss three major accounts put forward in philosophy of science to clarify or posit what exactly levels of organization are. These are Paul Oppenheim and Hilary Putnam’s “layer-cake” account, the mechanist account developed and advocated by Carl Craver (2007: ch. 5, 2015) and William Bechtel (2008: ch. 4), and William Wimsatt’s “local maxima” account (1976a, 1994). After these, we also go through recent skeptical approaches to the idea of levels of organization.