In this article, the authors first highlight the experiences in global monitoring and evaluation (M&E) coordination and collaboration. They then describe the collaboration experiences of two U.S. government agencies active in HIV prevention and care, the U.S. Agency for International Development (USAID) and the Centers for Disease Control and Prevention (CDC), and how they have participated in the global M&E partnership to set international standards and provide direct M&E support to national AIDS programs. As the main U.S. development agency, USAID works generally through country and regional missions to implement multisector programs, including preventive health and related services, through nongovernmental organizations. The CDC is the main U.S public health agency and part of the U.S. Department of Health and Human Services (DHHS), working both domestically and internationally. The CDC's international HIV/AIDS efforts operate generally through field offices that support the ministries' of health and their implementing partners. The CDC and USAID agencies have been working together intensively on HIV/AIDS M&E since 1999 and coordinating M&E financial and technical support to countries around the world. In 2004 they joined other U.S. government agencies in formulating one strategy for international HIV monitoring and evaluation. In fifteen HIV/AIDS focus countries, the agencies participate in a mission team that submits a yearly HIV/AIDS operational plan. The authors discuss the challenges faced and the successes achieved in harmonizing M&E strategies between the two agencies. They then describe the application of their M&E approaches within a real-life context in Cambodia, highlighting how these agencies work together on the ground to support the work of ministries of health, multisectoral national HIV/AIDS programs and councils, and a range of other governmental and nongovernmental organizations to strengthen implementation of M&E systems. The authors conclude by calling for enhanced coordination among "all" in-country partners within the commitment to the "three ones principle," which calls for "one" organizing authority, "one" national strategic plan, and "one" comprehensive national M&E plan. (Contains 1 figure.)