In many developed countries, needle exchange programs (NEPs) are a key component of harm reduction policies for people who use drugs. Most studies on NEPs suggest that they help prevent the transmission of the HIV virus, provided they are supplemented by other harm reduction measures. Their effectiveness in preventing the transmission of the hepatitis C virus (HCV) appears much less certain. Furthermore, it seems that young people who inject drugs (PWID), a group at high risk of HIV and HCV infection, rarely or never use NSPs. In addition to maintaining sufficient access to sterile syringes, it is necessary to consider how to reach young drug users who are beginning to inject drugs, particularly by engaging with their social networks. Efforts must also be made to promote prevention of risky sexual practices, which are strongly linked to injection drug use. Finally, the quality of evaluations of these programs must be improved.