This article argues that four distinct streams of thought flow into current conceptions of the knowledge economy. Drawing on these four streams of thought and on labor force data from the USA, the article develops a synthetic view of the knowledge economy. The analysis suggests that the knowledge economy, while of growing importance, remains smaller in the aggregate than other parts of the economy. The article also defines five sectors of the knowledge economy, according to the nature of the goods and services provided and the technology used in production. These sectors form an important basis for the segmentation and stratification of the professional labor force.