UNAIDS estimates that 800,000 children were infected with HIV in 2001, almost all through transmission of the virus from their mothers during pregnancy, childbirth, or breastfeeding. Of these, seven of each eight live in sub-Saharan Africa and most of the rest live in South and Southeast Asia. To combat mother-to-child transmission of HIV (MTCT), in 1999 the UNAIDS Secretariat, UNFPA, UNICEF, and WHO launched the Inter Agency Task Team on Prevention of HIV Transmission in Pregnant Women, Mothers, and Their Children (IATT), which provides guidance for prevention of MTCT (PMTCT). In the view of the IATT, PMTCT is part of broader strategies to prevent the transmission of HIV and sexually transmitted diseases, to care for HIV-positive women and their families, and to promote maternal-child health. The IATT has proposed a four-pronged approach for the prevention of HIV transmission to pregnant women, mothers, and their children, yet significant financial and technical challenges remain. To address these challenges, UNICEF has supported 11 pilot projects in Africa, Asia, and Latin America. This HIV/AIDS working paper provides guidance for improving and scaling up PMTCT programs, based on what program managers and evaluators identify as successful strategies