Community participation and leadership is a central tenet of public health policy and practice. Community engagement approaches are used in a variety of ways to facilitate participation, ranging from the more utilitarian, involving lay delivery of established health programmes, to more empowerment-oriented approaches. Evaluation methods within public health, adapted from clinical medicine, are most suited to evaluating community engagement as an 'intervention', in the utilitarian sense, focusing on the health impacts of professionally determined programmes. However, as communities are empowered and professional control is relinquished, it is likely to be harder to capture the full effects of an intervention and so the current evidence base is skewed away from knowledge about the utility of these approaches. The aim of this paper is to stimulate debate on the evaluation of community engagement. Building on current understandings of evaluation within complex systems, the paper argues that what is needed is a paradigm shift from viewing the involvement of communities as an errant form of public health action, to seeing communities as an essential part of the public health system. This means moving from evaluation being exclusively focused on the linear causal chain between the intervention and the target population, to seeking to build understanding of whether and how the lay contribution has impacted on the social determinants of health, including the system through which the intervention is delivered. The paper proposes some alternative principles for the evaluation of community engagement that reflect a broader conceptualisation of the lay contribution to public health.