Security force assistance (SFA) is a central pillar of the counterinsurgency campaign being waged by U.S. and coalition forces in Afghanistan. The outcome of the campaign hinges, in large measure, on the effectiveness of the assistance given to the Afghan National Army, Afghan National Police, and other security forces, assistance that the International Security Force must provide while fighting the insurgents. Yet senior U.S. military and civilian officials have posed many questions about the effectiveness of SFA in Afghanistan, and no empirically rigorous assessments exist to help answer these questions. This monograph analyzes SFA efforts in Afghanistan over time and documents U.S. and international approaches to building the Afghan National Security Forces from 2001 to 2009. Finally, it provides observations and recommendations that emerged from extensive fieldwork in Afghanistan in 2009 and their implications for the U.S Army.