The meaning and measures of structural unemployment are discussed in this paper. The first section examines the linkages between the concepts of cyclical, frictional, seasonal, and structural unemployment. The next sections analyze measures of structural unemployment based on the view that structural unemployment is substantial during times of high economic activity. Using the concept, the paper derives the distribution of the structurally unemployed. (Although the incidence of structural unemployment is higher among blacks, young, and ill-educated workers, the majority of the structurally unemployed is white, adults, and high school graduates.) The paper concludes by discussing the implications of the structural unemployment numbers for employment and training programs and for other government policies. (JH)