The biopsychosocial (BPS) model has been of considerable utility to those researching health and illness. This has been particularly the case for critical realists and those with a systemic orientation to their work. Whilst the strengths of the model are conceded in this article, its limitations are also examined. These relate to its ontological sophistication being compromised by its proneness to epistemological naivety. It is a model to explain the emergence of disease and disability, not a reflexive theory applicable to the whole field of health research. This tension is linked to the emergence of a re-orientation of medical knowledge away from bio-medical reductionism. From the outset the model was not about a general critique, or a grand theory, of health and illness. Instead it was a useful intervention, on the part of medical practitioners, like George Engel and Adolf Meyer, to create a reformed medical model on behalf of their profession. Case studies of the problematics of diagnosis and medicalization are offered to illuminate the limitations of the BPS model for health researchers.