One important collaborative tool developed by the S3 Platform to assist its member regions and Member States in this task is peer review. The S3 Platform views peer review as an important mutual learning and knowledge dissemination channel. The S3 peer review methodology was first developed and employed in 2012, yet it is being continuously improved and adapted to ensure the S3 Platform offers regions and Member States a framework that is structured enough to ensure regions under review receive adequate feedback, yet sufficiently flexible to facilitate open and productive discussions. While the European Commission uses peer reviews as a tool at a Member State level in a number of policy areas for some time now, it still appears to be an under-documented phenomenon in the regional policymaking context. This paper addresses this gap by documenting the newly developed S3 Platform peer review methodology.