This book is a part of the ongoing enterprise to understand the nature of human health and illness. This enterprise has expanded dramatically during the last decades. A great number of articles, as weIl as a fair number of monographs, on this topic have been published by renowned international publishers. In this discussion most participants share the idea that health is a partially normative concept, Le. that health is not a phenomenon which can be wholly characterised in biological (or otherwise descriptive) terms. To ascribe health to a person is eo ipso, according to this line of thought, to as­ cribe a positively evaluated property to this person. Moreover, most debators share the idea that health is a holistic property, belonging to the person as a whole, whereas diseases, injuries and defects are entities (or properties of entities) which can be very lim­ ited and and normally affect only a part of the individual. My own monograph belongs to this tradition. A feature of my position, which is not universally acknowledged in riyal theories, however, is my emphasis on the notion of ability as a fundament in the theory of health. In my formal characterisation of health I view it as astate of a person which is such that the person has the ability to fulfi1 his or her vital goals.