This paper makes the case for the establishment of a World Environment Organisation. It traces the development of the debate for a WEO over the past 40 years, and outlines the different models that have been proposed. Biermann argues that the transformation of UNEP into a Specialized Agency - either a World Environment Organisation or a UN Environment Organisation - would follow a long-standing policy of functional specialisation within the UN system and would significantly enhance the authority of UNEP. Such a set-up would allow the negotiation of environmental Treaties within 'sub-committees' of the UNEO General Assembly, which would then be adopted by the membership, thus moving away from the current fragmented system of Multilateral Environmental Agreements. It could also significantly enhance efficiency through the development of common reporting frameworks across multiple environmental regimes, thereby reducing the reporting burden on developing countries. The development of a WEO is presented not as a 'silver bullet' but as an important building block for the enhanced effectiveness of earth system governance.