This Review of the Initial Impact of the GEO-4 Report, conducted ten months after GEO-4’s launch, was designed to assess the extent to which target audiences have used the global GEO-4 Report and the Summary for Decision Makers, the influence it has had to date, and to identify the main factors affecting its use and influence. The premise of the Review is that the extent and types of use and influence are good indications of the potential impact of GEO-4 over time, and that understanding the patterns and the reasons will highlight issues that need to be considered in the design of GEO-5. Current literature on leading global assessment processes, previous assessments, the project logic and a mixed methods approach were used to capture and analyze the perceptions and experiences of just over 150 users and potential users, as well as key UNEP staff. Surveys and Internet research provided additional insights beyond GEO’s ‘inner circles’.