Endometriosis is a common disorder of women of reproductive age, yet diagnosis of this condition is often problematic. The most frequent clinical presentations of endometriosis include dysmenorrhea, pelvic pain, dyspareunia, infertility, and pelvic mass. However, the correlation between these symptoms and the stage of endometriosis is poor. Currently available laboratory markers are of limited value. At present, the best marker, serum CA-125, is usually elevated only in advanced stages and therefore not suitable for routine screening. Transvaginal ultrasound and magnetic resonance imaging are often helpful, particularly in detection of endometriotic cysts. Recently, transrectal ultrasound and magnetic resonance imaging were shown to be valuable in detection of deep infiltrating lesions, especially in the rectovaginal septum. Although direct assessment of endometriotic foci at laparoscopy may be viewed as a "gold standard" for identifying endometriosis, the correlation of laparoscopic observations with histological findings is often low. Ultimately, diagnosis of endometriosis requires a careful clinical evaluation in combination with judicious use and critical interpretation of laboratory tests, imaging techniques, and, in most instances, surgical staging combined with histological examination of excised lesions.