The mission of the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) is to safeguard the health of low-income women, infants, and children through age 4 who are at nutritional risk. WIC provides nutritious foods to supplement diets, nutrition education, and referrals to health care and other social services. Administered by USDA's Food and Nutrition Service (FNS), almost half of all infants and about a quarter of all children ages 1-4 in the United States participate in the program. WIC is USDA's third-largest food and nutrition assistance program, accounting for 10 percent of total Federal spending on food and nutrition assistance. This report describes the WIC program--how it works, its history, program trends, and the characteristics of the population it serves. It also examines current issues facing WIC, focusing mainly on those with important economic implications. Methodology for Estimating WIC Participants as a Percent of U.S. Population Subgroups is appended. (Contains 23 figures, 7 tables, and 126 footnotes.)