Cooperation is an important way that decision centres interact in a polycentric governance system. Cooperation in governance has been studied by numerous scholars in the field of 'collaboration', although such scholarship seldom explicitly sets it within the framework of polycentricity. Cooperation involves multiple decision centres working across boundaries to pursue shared goals, and it is especially prevalent for addressing complex socio-ecological systems. This chapter examines cooperation in the Puget Sound basin, USA, for ecosystem restoration. This chapter describes how authority, information, and resources affected cooperation in formation of the Puget Sound Partnership and related Local Integrating Organizations, development of ecosystem recovery plans, and implementation of ecosystem recovery projects. It assesses polycentric governance performance in terms of outcomes and processes. The Puget Sound ecosystem restoration efforts exhibit relatively high levels of coherence, representation, and adaptability; relatively low levels of efficiency and accountability; and mixed results on efficacy and network building.