BACKGROUND: Empirical evidence demonstrates that the Thai Universal Coverage Scheme (UCS) has improved equity of health financing and provided a relatively high level of financial risk protection. Several UCS design features contribute to these outcomes: a tax-financed scheme, a comprehensive benefit package and gradual extension of coverage to illnesses that can lead to catastrophic household costs, and capacity of the National Health Security Office (NHSO) to mobilise adequate resources. This study assesses the policy processes related to making decisions on these features. METHODS: The study employs qualitative methods including reviews of relevant documents, in-depth interviews of 25 key informants, and triangulation amongst information sources. RESULTS: Continued political and financial commitments to the UCS, despite political rivalry, played a key role. The Thai Rak Thai (TRT)-led coalition government introduced UCS; staying in power 8 of the 11 years between 2001 and 2011 was long enough to nurture and strengthen the UCS and overcome resistance from various opponents. Prime Minister Surayud's government, replacing the ousted TRT government, introduced universal renal replacement therapy, which deepened financial risk protection.Commitment to their manifesto and fiscal capacity pushed the TRT to adopt a general tax-financed universal scheme; collecting premiums from people engaged in the informal sector was neither politically palatable nor technically feasible. The relatively stable tenure of NHSO Secretary Generals and the chairs of the Financing and the Benefit Package subcommittees provided a platform for continued deepening of financial risk protection. NHSO exerted monopsonistic purchasing power to control prices, resulting in greater patient access and better systems efficiency than might have been the case with a different design.The approach of proposing an annual per capita budget changed the conventional line-item programme budgeting system by basing negotiations between the Bureau of Budget, the NHSO and other stakeholders on evidence of service utilization and unit costs. CONCLUSIONS: Future success of Thai UCS requires coverage of effective interventions that address primary and secondary prevention of non-communicable diseases and long-term care policies in view of epidemiologic and demographic transitions. Lessons for other countries include the importance of continued political support, evidence informed decisions, and a capable purchaser organization.