The long-term development of irrigation in Thailand and its effect on rice yields are analyzed, using data from the period 1961-69. After providing an overview of the historical development of irrigation in Thailand, the report examines the status and performance of irrigation projects in terms of area irrigated and rice produced. Different categories of state irrigation systems are set forth and irrigation projects carried out by local populations described. The regional distribution of irrigated lands is assessed and types of irrigation work by region are identified. Next, wet-season yields in irrigated and rain fed areas are compared. It is noted that while irrigated yields have surpassed rain fed yields, the gap between them has declined since 1968-69. The costs of constructing, operating, and maintaining large, medium, and small irrigation projects and land consolidation and ditches and dikes projects are then discussed. Finally, prospects for addressing the irrigation objectives of the Government of Thailand's 1982-86 national development plan are assessed. It is concluded that rice yields are likely to increase as a result of expansions of both irrigated and rain fed areas. Twenty-two tables illustrate the text.