--- - |2 Abstract Rural policy evaluation helps to understand the extent to which policies have met pre‐defined objectives, achieve value for money and learn from implementation failures. However, there is increasing debate over the quality of policy evaluation and the extent to which its methods can fully contribute to an understanding of rural policy. Responding to these calls, this article employs a theory driven approach to policy evaluation to assess the social impacts of attempts to reduce animal disease on farms in England. Popular in other policy arenas, theory driven evaluation relies on developing a theory of change to examine the interactions between policy contexts and mechanisms and policy outcomes and determine what works for whom. Drawing on longitudinal qualitative and quantitative research, the article identifies two mechanisms of change to evaluate the Badger Vaccine Deployment Project (BVDP) in England to reduce incidence of bovine Tuberculosis (bTB) in cattle. The article shows how these mechanisms – ‘seeing is believing ' and ‘practice similarity' – are triggered by different contextual factors leading to the failure to deliver expected policy outcomes. We also consider the advantages and limitations to theory based evaluation, and the contribution it can make to the evaluation of other rural development programmes. - 'Sociologia Ruralis, EarlyView. '