The paper argues that Thailand's economic and social development from the late 19th century to the early 21st century presents a puzzle. For much of the period from 1870 to 1940, the country's economic growth was slow, and the economy remained agricultural, with little diversification into modern industry or services. It was the only Southeast Asian country to escape direct colonization, and yet it did not use its relative freedom from colonial control to embark on a programme of accelerated economic, social and political modernization. The contrast with Meiji Japan has been made by several Thai and foreign scholars, but Thailand's growth was also slow in comparison with several neighbouring countries under colonial control. Only in the late 1950s did economic growth start to accelerate and by 1996, per capita GDP was well ahead of other ASEAN countries except Malaysia and Singapore. The paper explores the reasons for the accelerated growth, looking particularly at the role of government. The paper also examines the reasons for the growth collapse of 1997/1998, and the slower economic growth since then.