Partnerships are increasingly seeking tools that enable stakeholders to reflect on their own effectiveness, benchmark the status of their partnership and provide a framework for development. Drawing on the evaluation of two Health Action Zones, this article focuses on the use of one such formal assessment tool, adapted from the Nuffield Partnership Assessment Tool and the Verona Benchmark, to explore the contribution of formal tools to our understanding of partnership. It outlines some key methodological limitations and stresses the continued importance of an understanding of context alongside any measurement of partnership effectiveness. It is suggested that formal assessment tools can be extremely valuable in terms of the learning that can result both from the process itself and from the outcomes of the assessment. However, as a stand-alone device they are open to misinterpretation and unlikely to foster development other than in those partnerships prepared to invest the necessary resources in a broad-based evaluation.