The notion of health used in medicine may have important implications, such as guiding the allocation of medical resources. This paper explores the notion of health through an overview and conceptual analysis of various notions of health found in modern medical and philosophical literature. It argues that health is characterized either positively or negatively (per exclusion), and either mechanistically (as the set of common or ideal states of a species) or holistically (as unimpaired self-organization of organisms). The paper concludes that a sound notion of health characterizes health negatively and holistically, assimilating mechanism as a good approximation in simple cases.