Chlamydia trachomatis infection is the most common sexually transmitted disease all around the world. For a long time, Chlamydia trachomatis has been recognized as the common cause of urethritis, cervicitis, and other complications such as pelvic inflammatory disease (PID), ectopic pregnancy, etc. However, today there is more concern on its role in male and female fertility and infertility and many studies have concentrated on this issue all over the world. There is good evidence based on seroepidemiologic studies on relationship between serum Chlamydial antibodies and infertility due to tubal factor in women with or without PID. However, there are many questions about Chlamydia trachomatis and its influence on IVF outcome and the exsisting information is controversial. The impact of Chlamydia trachomatis on male infetility is under investigation and it seems able to cause destruction of different parts of male genital tract in addition to high rate of transmission to women. With regard to these data, it seems that prevention, early diagnosis and treatment of Chlamydial infections are importart and cost effective.