The nature, normativity and definition of health and disease are one of the major issues in the philosophy of medicine. First proposed in 1975, the biostatistical theory of Christopher Boorse has been central in shaping the debate between the naturalist and normativist views of these concepts and this theory still provides the main naturalist definition of health. But what exactly is meant by naturalism in the BST, and more generally in the philosophy of health, is far from clear. Further, over the past few years, interest in the naturalist stance has been strongly renewed. The time has thus come to re-assess the relevance and status of naturalism in the analysis of health concepts, as well as its implications for healthcare and the debate on health enhancement. Three main reasons for this reassessment are considered: (1) clarifications of the BST, as well as possible improvements, have been proposed by several authors; (2) there could be other forms of naturalism in the philosophy of health than that of the BST: the notion of ‘biological normativity’, initially coined by Georges Canguilhem, has seen some revived interest, along with the recent development of ‘organizational approaches’ to biological function; (3) more needs to be said about the utility of and reason behind philosophical efforts to seek a definition of health concepts. These three main reasons correspond roughly to the three parts of the volume, which are introduced in this opening chapter.