This paper considers the implications of mass communications theory on public relations (PR) evaluation and briefly reviews mass communication effects, persuasion, and cognition, attitude and behaviour change theories. The implications for evaluation are then examined. Reliance on domino models is shown to be too simplistic. It is suggested that claims of PR behavioural effects may be unrealistic and it is argued that more moderate and/or alternative goals are needed if preordained failure is to be avoided. Evaluation results must be interpreted cautiously so that further significance that is not supported by theory is not assumed. This paper shows how the concept of PR evaluation could be widened to include formative evaluation and broad environmental monitoring, which are especially important in identifying and understanding why and how communication works, what its effects are, what factors restrict or facilitate effectiveness and under what conditions success can be maximised.