What parameters determine brain size? This question is of particular interest for humans because our large brains confer outstanding cognitive abilities. The answer has long been sought in comparative analyses of brain size relative to body size (herein termed ‘brain size’) in our fellow homeothermic vertebrates – namely other mammals and birds. Unfortunately, brain size is an idiosyncratic trait corresponding to a seemingly miscellaneous collection of traits ranging from gestation length to deception behaviour. Some order can be established by attributing (...) brain size correlates to categories of constraint (‘what traits permit or limit increased brain sizes?’) and selection (‘what traits select for increased brain sizes?. Even so, the vast number of potential correlates has lead to a plethora of (frequently mutually exclusive) hypotheses regarding avian and mammalian brain size evolution. This confusion has recently prompted calls for better integration of brain size correlates. In this article we argue that the latest advances in the field of brain size constraints, combined with modern techniques for tracing brain development, put an integrated framework of brain size evolution within our reach. (shrink)