The paper examines the nature and development of the participatory water institutions in the eastern Indo-Gangetic plains of India, focusing on the aspects of structure, participation, and devolution. Though the physical development of irrigation has made considerable progress in India, the proper management and distribution of water has poised many difficulties. The consequences of this are poor efficiency in water use, inequity in distribution, disputes, high cost, and substantial under-utilization of the potential created. On the other hand, institutional initiatives that aim to improve water management and distribution are seen in some areas/locations, and show a process of arriving at better institutional arrangements. Water institutions are crucial for eastern India and though there are a few examples of spontaneous bottom-up initiatives, much of this development is driven by external interventions including laws, policies, and government programs. Even though under the government interventions, the guidelines and policies are usually uniform and top-down, the local uptake and adoption show substantial variation and divergence and this deserves study. The research is based on review of the literature as well as on six in-depth case studies, and also responses of 510 households involved with 51 participatory water institutions in the setting of the eastern Indo-Gangetic plains states of Assam and Bihar. The paper first takes a brief look at the literature on the foundations and experiences of participatory irrigation management (PIM), and then examines through the case studies and data, the development and variation in the PIM water institution in the given setting. It examines features such as laws, membership, structure, inclusion, participation/involvement and devolution. It finds that inclusion of various groups of people in the institutions is quite good except for women and youth. However, actual involvement of different people varies substantially. The issue of devolution/decentralization versus centralization in decision-making is very important to PIM, and varies across the structure and functions. The association of involvement and devolution to performance indicates that the active involvement of some functionaries and groups is very important, and that devolution in several decisions can considerably enhance performance. The observations provide many useful insights for policy and institutional design which can help improve water resource management in the eastern Indo-Gangetic plains.