STDs (sexually transmitted diseases) refer to a variety of bacterial, viral, and parasitic infections that are acquired through sexual activity including vaginal intercourse, oral sex, and anal sex. The term sexually transmitted infection (STI) is used increasingly because it has a broader range of meaning; a person may be infected and may potentially infect others without showing signs of diseases. Approximately 340 million new cases of STDs occur each year in the world. According to the Korea Centers for Disease Control and Prevention, notified cases of STDs in Korea decreased from 32,872 in 2002 to 12,486 in 2007. Bacterial STDs such as gonorrhea and chlamydial infection declined continuously but syphilis and chlamydial infection increased again. Viral STDs such as genital herpes and condyloma acuminatum increased continuously. Gonorrhea and chlamydial infection are still the two most commonly reported STDs in Korea. Bacterial STDs can be cured. However, the problem is that viral STDs cannot be cured completely at present. Safe sex with proper education and the correct use of condom are important to prevent STDs and unwanted pregnancy. Early diagnosis and screening, proper treatment, and appropriate counseling from qualified physicians are necessary to prevent the complications and spread of STDs. Also, continuous monitoring and strategies of prevention of STDs will be necessary for improvement of the public health.