For two decades Thailand's National Medicinal Drug Policy has sought to make effective and safe drugs of good quality available and affordable to all, and to promote the rational use of drugs. Using original policy documents, secondary sources and interviews with Thai informants, the history of the development and implementation of the policy are examined and salient issues associated with primary health and medicinal drugs in Thailand are identified. It is argued that the policy improved the accessibility and quality of essential drugs. However, further action is needed to reduce the level of irrational usage, to foster the use of traditional medicines and to encourage national self-reliance in the production of essential drugs. This discussion of policy development provides lessons for other countries working to establish their own medicinal drug policies.