An ecosystem assessment is a social process through which the findings of science concerning the causes of ecosystem change, their consequences for human well-bring, and the management and policy options are evaluated. Ecosystem assessments can play an important role in synthesising and communicating complex information and can both inform and influence decision-making processes. This paper draws insights from experience with National Ecosystem Assessments (NEAs) recently undertaken in the UK, Japan, Spain, and Portugal, as well as other ecosystem assessments undertaken at regional and international geographical scales, and highlights lessons learned so that the impact of NEAs on policy can be enhanced. The paper concludes by identifying key issues needed to develop practical guidance for successful ecosystem assessments.