Formulating government policies involves a wide network of stakeholders and policymakers and numerous techniques and strategies. Moving from policy formulation to policy evaluation, a key challenge is to enable the effective involvement of this network in the evaluation activities that aim to ex- amine the implementation and impact of a public policy. This paper studies the process of evaluating the quality of policy formulation and the effectiveness of its implementation introduced by the Open Government Partnership, a global open government initiative, and illustrates related aspects and challenges. Building on this study, the paper proposes a generalised open and inclusive evaluation model that may be applied for assessing the effectiveness of public policy development and implementation. Although based on the domain of open government, the proposed co-evaluation model may be of wide applicability to other public policy domains thus supporting the new role of government ‘as a platform’.