The standard models used in the study of complex public health interventions are inadequate. They adopt a simple empiricist theoretical foundation and attempt to graft onto an essentially open social system a contrived laboratory experimentation typically in the form of a randomized, controlled trial. By understanding the ontological and epistemological claims of critical realism, it is possible to transcend the methodological inadequacy of the standard model approach. Critical realism posits a substantive causal theory, an end to fact-value dualism, and a coherent and emancipatory model of social action; all of these features amount to a systematic and compelling account of public health practice and a coherent approach to evaluation of complex public health interventions.