 We mentioned above that the reduction of Mendelian genetics to molecular genetics was one of the first topics to be discussed in the philosophy of biology. The initial debate between Schaffner and Hull was followed by the so-called “anti-reductionist consensus” (Kitcher 1984). The reductionist position was revived in a series of important papers by Kenneth Waters (1990, 1994) and debate over the cognitive relationship between the two disciplines continues today. However, the question is not now framed as a simple choice between reduction and irreducibility (Griffiths 1999; Kitcher 1999; Sober 1999). Rather, a variety of concepts have been introduced and discussed including multiple realizability, realization, and mechanism (Wilson 2005, Craver 2007; Polger &amp; Shapiro 2016). These have been important in thinking about genetics but also neuroscience and cell biology more generally. For example, Lindley Darden, Schaffner and others have argued that explanations in molecular biology are not neatly confined to one ontological level, and hence that ideas of ‘reduction’ derived from classical examples like the reduction of the phenomenological gas laws to molecular kinematics in nineteenth century physics are simply inapplicable (Darden and Maull 1977; Schaffner 1993). Moreover, molecular biology does not have the kind of grand theory based around a set of laws or a set of mathematical models that is familiar from the physical sciences. Instead, highly specific mechanisms that have been uncovered in detail in one model organism seem to act as “exemplars” allowing the investigation of similar, although not necessarily identical, mechanisms in other organisms that employ the same, or related, molecular interactants. Darden and others have argued that these “mechanisms” – specific collections of entities and their distinctive activities – are the fundamental unit of scientific discovery and scientific explanation, not only in molecular biology, but in a wide range of special sciences (Machamer, Darden et al. 2000; Craver &amp; Darden 2013; see also Bechtel and Richardson 1993). There is an interesting question about whether the notion of mechanism aptly applies in other areas of biology (Skipper &amp; Millstein 2005; Havstad 2011). 