This report provides information on the participation patterns of infants and children who were enrolled in the Special Supplemental Nutrition Program for Women, Infants, and Children, known as WIC, from Fiscal Years (FY) 2001 through 2003. Information on the dynamics of WIC participation leads to a better understanding of overall trends in the WIC caseload and the factors that affect participation. The study focused on four aspects of WIC participation by infants and children—entry, exit, continuity of participation, and re-entry. These events were examined in relation to the age of the infants and children, the characteristics of their mothers, and other broad socioeconomic factors. In FY 2001 through FY 2003, WIC served an average of 5.7 million infants and children and 1.8 million mothers each month. Average monthly participation has grown to 6.5 million infants and children and 2.2 million mothers in FY 2008. The proportion of total participants who are infants and children has been consistently around 75 percent.