Published works on health insurance tend to focus on program design and its impact, neglecting the implementation process that links the two and affects outcomes. This paper examines the National Health Insurance [Rashtriya Swasthya Bima Yojana (RSBY)] in India with the objective of assessing the role of implementation structures and processes in shaping performance. The central question that the paper addresses is: why does the performance of RSBY vary across states despite similar program design? Using a comparative case study approach analyzing the program's functioning in three states, it finds the answer in the differences in governance of implementation. The unavoidable gaps in design of health care program allow abundant scope for opportunistic behavior on the part of different stakeholders. The study finds that the performance of the program, as a result, depends on the extent to which the governance mechanism is able to contain and channel opportunistic behavior during implementation. By opening up the black box of implementation, the paper contributes to improving the performance of national health insurance in India and elsewhere.