Sexually transmitted diseases (STD) are now the commonest group of notifiable infectious diseases in most countries, particularly in the age group of 15 to 50 years and in infants. Their control is important considering the high incidence of acute infections, complications and sequelae, their socioeconomic impact, and their role in increasing transmission of the human immunodeficiency virus (HIV). The worldwide incidence of major bacterial and viral STD is estimated at over 125 million cases yearly. STD are hyperendemic in many developing countries. In industrialized countries, the bacterial STD (syphilis, gonorrhoea, chancroid) declined from the peak during the Second World War till up to the late fifties, then increased during the sixties and early seventies, and they have been decreasing again from the late seventies till the present. In the industrialized world, diseases due to Chlamydia trachomatis, genital herpes virus, human papillomaviruses and human immunodeficiency virus are now more important than the classical bacterial ones; both groups remain major health problems in most developing countries. Infection rates are similar in both women and men, but women and infants bear the major burden of complications and serious sequelae. Infertility and ectopic pregnancies are often a consequence of pelvic inflammatory disease, and are preventable. Sexually transmitted diseases in pregnant women can result in prematurity, stillbirth and neonatal infections. In many areas 1-5% of newborns are at risk of gonococcal ophthalmia neonatorum, a blinding disease; congenital syphilis causes up to 25% of perinatal mortality. Genital and anal cancers (especially cervical cancer) are associated with viral sexually transmitted diseases (genital human papillomavirus and herpes virus infections). Urethral stricture and infertility are frequent sequelae in men.