The sexually transmitted infections (STIs), also known as sexually transmitted diseases (STDs), are caused by infectious agents spread by any sexual activity (vaginal intercourse, anal sex, and oral sex). For some of these infections, a nonsexual transmission is possible, e.g., vertical transmission, blood, and donor tissue. With an estimated incidence of more than one million new infections per day, it is a critical public health issue because of the severe long-term STI-related complications, such as pelvic inflammatory disease (PID), infertility, neurological and cardiovascular disease, malignancies, pregnancy issues, till neonatal death. STIs can cause urethritis and cervicitis (chlamydia, Neisseria gonorrhoeae), vaginal discharge (bacterial vaginosis, aerobic vaginitis, candidiasis, trichomoniasis), and genital ulcerations (syphilis, herpes virus). Nonetheless, a very common aspect among STIs is the absence of symptoms in a consistent part of the cases, leading to underestimation, to possible permanent sequelae, and to an increased rate of transmission. Prevention is the most effective way of reducing risk for STI acquisition. Main interventions include the assessment of risk factors in the population, education and awareness, and behavioral modifications such as condom use. For some STIs, there are currently licensed vaccines, such as those against HPV or HBV.