During economic development, agriculture declines as a proportion of aggregate national output. A number of theoretical explanations for this phenomenon have been advanced in the economic literature, but their relative historical significance has not been clear. This paper develops an econometric methodology to analyze this issue and applies it to time series data for Thailand. The study investigates the importance of relative commodity prices, factor endowments and technical progress as explanations for changing sectoral GDP shares of agriculture, manufacturing and services. It is concluded that the movement in Thailand's aggregate factor endowment relative to labor was the most important determinant of agriculture's relative decline.