Sexually transmitted infections (STIs) are caused by more than 30 species; the organisms with the highest frequency and clinical importance are Chlamydia trachomatis, Neisseria gonorrhoeae, Treponema pallidum, Mycoplasma genitalium, the hepatitis B and C viruses, the human immunodeficiency virus, the human papillomavirus, the herpes simplex virus type 1 and type 2 and Trichomonas vaginalis. Most of these infections are present without symptoms, and when they are present, the most common clinical manifestations of sexually transmitted diseases include vaginal discharge, genital ulcers and abdominal pain in women. Worldwide more than 1 million people acquire an STI per day, and an estimated 500 million people are infected each year with one of the next four STIs: chlamydia, gonorrhoea, syphilis or trichomoniasis. More than 290 million women are infected with human papillomavirus, and more than 530 million people have the virus that causes herpes simplex virus type 2 at this time. Such infections can have serious consequences beyond the immediate effects of the infection itself, that is transmission from mother to child during pregnancy and childbirth, and chronic diseases. Given the importance of these illnesses, in this review, we present the most important aspects of the causative agents, diseases, epidemiology and diagnosis of STIs.