Multilateral Development Banks (hereinafter MDBs) have evolved from avoiding non-economic considerations, as required in most of their charters, to officially supporting sustainable development. This transformation is due not only to international law, civil society pressure or internal adjustments, but in part to the emulation phenomenon between the international effort for sustainable development made by international conferences and commissions and the sustainable development effort by MDBs. The paper first examines this emulation phenomenon and its strong impact on MDB creation of substantive instruments – the environmental and social safeguards – and of procedural instruments – the accountability mechanisms – for the integration of sustainable development. Following this discussion, the second part points out that MDBs’ environmental and social safeguards still differ substantially, revealing a fragmentation in MDBs’ normative understanding of sustainable development. After presenting the arguments supporting this fragmentation, the last part of the paper argues in favor of the harmonization of MDBs’ environmental and social safeguards, resulting in the creation of a common law of sustainable development, the promotion of international law and the facilitation of investments.