This chapter assesses the contribution of immigrant workers to GDP in Thailand, based on labour market and other information. Assuming that the economic contribution of immigrant workers is broadly related to the number of workers, it is possible to make a quantitative assessment of the direct output generated by immigrants in Thailand. As the sectoral distribution of workers is a major determinant of the contribution to GDP, this chapter starts with a brief review of sectoral development of the Thai economy and the position of foreign-bom workers in this regard.