This chapter examines the case for assigning high priorities in developing countries to prevention of sexually transmitted diseases (STDs) both the "classic" diseases and HIV/AIDS. The distinctive epidemiology and prevalence of the 20 classic STDs and AIDS is that while they may be bacteria virus protozoa fungi or ectoparasites they are all specifically adapted to the genital tract and transmitted primarily by sexual intercourse. The highest rates of infection are in young urban socially mobile or unemployed adults aged 15-35. The STDs are highly communicable with a high "reproduction rate" such that prevention of one case had "dynamic benefits" in preventing subsequent cases. Sexual activity can be subdivided into a "core group" of highly sexually active individuals (about 2% of the population 10 times as sexually active with new randomly selected partners every 5 days) and "non-core" group. The epidemiology i.e prevalence complications and sequelae of classic STDs and AIDS in developing countries is reviewed. The public health impact is discussed in terms of years of healthy life lost per case in comparison to other major diseases. In a typical African city 22% of total disease burden is due to STDs and HIV. The static dynamic short and long term and gender-specific burdens and demographic impact of these diseases are derived theoretically. One prediction by Bongaarts model projects that the population growth rate will fall from 3% to 1.9% as a result of AIDS in 25 years. Examples of gender-specific burdens are infertility and divorce of affected women and discontinuation of contraception because of blaming genital discharge on contraceptives. Principles of primary prevention of STD are discussed under the topics of static and dynamic benefits and costs of averting a case sexual transmission vertical transmission transfusion and skin-piercing transmission. A policy of targeting by one-time intervention at the core averts 10 times as many cases as would have been averted by a policy directed at the non-core. Mandatory prevention such as detaining core individuals is best suited for emergency control of politically unpopular behavior. Voluntary prevention can better be achieved by changing prices e.g. improving the marriage education and job opportunities for women who would otherwise engage in prostitution as has the "enterprise zone" in northern Mexico. The goals principles costs future developments priorities and potential benefits of case management of STDs are reviewed. The control and management of STDs when the core group is targeted is highly cost-effective compared to non-targeting. Appendices include health impact of individual STDs and clinical management of STD syndromes.