The currently dominant model of health and disease in psychiatry and medicine is Engel’s biopsychosocial (BPS) model, proposed in the 1970s to advance reductionistic biomedicine by integrating psychological and social factors. Although the BPS model represented progress, its scientific and philosophical foundations remain questionable and it cannot be considered complete or sufficient. In this chapter, we provide a historical and conceptual analysis of the BPS model before showing that the integration of evolutionary theory can provide a suitable next step from the BPS model, much as the BPS model was a step forward from the biomedical approach. Evolutionary theory justifies and enhances the BPS model’s recognition of multiple levels of causation and expands it by recognising both ultimate and proximate causation. It allows a clearer distinction of biological function from dysfunction and encourages a phylogenetic perspective on biology, which can guide research in new directions. In connecting the model of health with the most fundamental theory of biology, this approach provides the philosophical and scientific coherence that the BPS model sorely lacked.