Every four years, the Global Environment Facility (GEF) is replenished by its donors. Each replenishment process has been informed by independent overall performance studies of the GEF. These studies have developed into authoritative reviews of the state of the art and of available knowledge on the functioning and results of the GEF. Previous reviews were undertaken by teams of independent experts; this Fourth Overall Performance Study (OPS4) was, for the first time in the study series’ history, undertaken by a GEF entity itself: the GEF Evaluation Office.