Virtually all sizes and types of farms employ farm labour. During the last decade, approximately half of the farm labour force has been made up of unauthorised workers. The purpose of this paper is to analyse the unique aspects of farm labour markets that make labour shortages possible in the face of high levels of unemployment, and what appears to be a persistent flow of undocumented workers, largely from Mexico, into the United States. The paper hypothesizes that the hired farm labour workforce comprises at least three distinctly different subsectors composed of a mix od seasonal and non-seasonal workers. These segments include: (1) fruit and vegetable workers; (2) workers employed on field crop farms; and (3) workers employed in animal agriculture. We further hypothesize that farm labour markets are local, not regional or national. Responsible analysis of how changes in immigration policy will impact stakeholders will require the use of correct definitions of farm labour markets.