The Global Water Partnership (GWP) represents a new kind of institution that raises various interesting questions in terms of international institutional law. Established in 1996 as a 'virtual organization', it progressively evolved to become in 2002 a twofold institution structured around, on the one hand, a network without legal personality and, on the other hand, an international organization with full legal personality under international law. This article aims to analyze this unique structure and its consequences on membership and organic issues. The reasons underpinning this evolution will also be studied. If institutional flexibility prevailed in the early life of the institution, practical problems and concerns about legal legitimacy have led to a more formal structure in its later life. Principles such as those developed by the Global Administrative Law (GAL) project, namely legitimacy, transparency and accountability, have been also widely used to shape and organize this structure.