Summary: GAO examined the performance measures that the Food and Nutrition Service (FNS) uses to assess the nutrition education, breastfeeding promotion and support, and health referral services provided to participants in the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) program. GAO found that FNS has an outcome-based measure for one of the three nutrition services--breastfeeding promotion and support. However, the measure, breastfeeding initiation rate, examines only one of several important aspects of the service's possible impact on WIC participants. In addition, several obstacles have hindered FNS' efforts to develop and implement outcome-based measures for nutrition education and health referral services for the WIC program. These include difficulties in identifying measures that would allow the agency to appropriately link a particular service's activity to a desired outcome and resource constraints affecting FNS' ability to collect data needed to implement a proposed measure.