The World Bank Group was a principal founding partner of the Global Environment Facility (GEF) in its pilot phase in 1991, and of the restructured GEF in 1994. The Bank plays three different roles in the GEF: (a) as trustee of the GEF and related trust funds, (b) as implementing agency, including the implementation of private-sector GEF projects by the International Finance Corporation (IFC), and (c) as the host organisation of the functionally independent GEF secretariat. Focusing primarily on the role of the Bank as an implementing agency, this review documents how the partnership that the GEF and the World Bank Group established in the early 1990s has evolved over time, offers explanations for observed changes, and draws a number of lessons. The review addresses the following issues: (i) The mutual relevance of the World Bank Group and the GEF; (ii) Inter-organisational coordination along the World Bank Group-GEF project cycle; (iii) The introduction of the GEF's resource allocation systems in 2006 and 2010; (iv) The evolution and effectiveness of the Bank Group's GEF portfolio; (v) Catalytic approaches in the Bank Group-GEF partnership: co-financing, blending, and mainstreaming; (vi) The World Bank's corporate activities as a GEF implementing agency. The principal purposes of this review are (a) to help improve the relevance and effectiveness of the Bank Group's partnership with the GEF, and (b) to draw lessons for the Bank Group's partnership with the GEF and other large global partnership programs.