Head Start, created in 1965, is designed to prepare low-income preschool children for school by providing a comprehensive set of early child development services primarily through community based organizations. Over the last decade there have been a number of changes in Head Start's operating environment, including a decrease in the number of poor children; an increase in the number, size, and scope of other federal and state early childhood programs; and an expansion in Head Start spending and enrollment. Given this environment, GAO was asked to determine: (1) what is known about the extent to which Head Start programs are underenrolled; (2) the Administration for Children and Families (ACF) regional officials' and Head Start grantees' views on what factors contribute to underenrollment; and (3) what actions ACF and grantees have taken to address underenrollment. (Contains 5 appendices, 9 tables, and 5 figures.)