This article presents harm reduction, which was recently developed in response to the expansion of injection drug use and the HIV/AIDS epidemic. The authors describe the essence of harm reduction, proposing that harm reduction is a pragmatic strategy from a public health perspective to deal with injection drug use. Also, how harm reduction complements the traditional strategies to eradicate illicit drug use based on abstinence (so called, War on Drugs, or Just Say NO) is discussed. By describing key principles of harm reduction, such as low-threshold programs, non-judgmental attitudes, priority of immediate goals, and balancing costs and benefits, the authors introduce major harm reduction programs, which include needle/syringe exchange, outreach, counseling and education, supervised injection sites, and substitution treatment. Substantial evidence demonstrates harm reduction is effective in preventing the spread of HIV. Although international bodies, such as UNAIDS and WHO, advocate harm reduction strategies for the better prevention from the spread of HIV/AIDS, and some countries have adopted national harm reduction policies, United States discourages harm reduction policies in fighting the global HIV/AIDS pandemic. Finally, the authors address the effectiveness of harm reduction from the public health perspectives to deal with AIDS epidemic among injection drug users and the necessity of comprehensive understanding and multifaceted application of harm reduction. They also present the need to rethink Japanese government policies and social programs to meet drug users' health needs.