In a public health framework, primary prevention means reducing the number of new instances of intimate-partner violence and sexual violence by intervening before any violence occurs. Program and research in primary prevention has lagged efforts in secondary and tertiary prevention, which focus on people who are at risk or already have suffered violence. This background paper helps to close that gap and is the basis for a guideline on intimate partner and sexual violence prevention currently being prepared by WHO, CDC, and the London School of Hygiene and Tropical Medicine.