The composition of national output-that is, the shares of income origi­ nating in various industries-has important implications for overall labor productivity and real economic growth. Within the last seventy-three years, there have been some significant changes in the makeup of U.S. output, partly secular and partly cyclical. These shifts have been dictated by alterations of the mix of goods and services demanded by consumers and by the government. The following analysis of specific changes in the composition of demand and output and their relationship to productivity and income growth should broaden our understanding of the past, pres­ ent, and prospective path of U.S. economic growth.