Both philosophers and scientists make use of the distinction between ultimate and proximate explanations, but its precise content remains unclear. Those philosophers who have carefully examined the distinction have not yet reached a consensus on the kind of explanations that ultimate and proximate explanations are, nor what sets them apart. This paper scrutinizes the characteristics of the proximate/ultimate distinction in contemporary accounts of human cooperation, both to highlight key features of these explanations as well as to clarify the distinction itself. We argue that in practice it is hard, if not impossible, to identify a set of features that neatly sets the two kinds of explanation apart, but that each kind can nonetheless be characterized by a cluster of loosely-related characteristics.