Although laparoscopy is the gold standard for the diagnosis of endometriosis, the need for this invasive diagnostic tool is a major stumbling-block in both effective clinical management and for research into this common and debilitating disease. As visual inspection of the pelvis has also major limitations, particularly for the diagnosis of posterior pelvis, bowel and bladder endometriosis, it is not surprising that considerable efforts are being made to improve the diagnosis by imaging techniques. Peritoneal endometriosis and ovarian endometriomata are predominantly haemorrhagic lesions. During laparoscopy, these lesions are readily identified by the presence of old or recent bleeding. Both transvaginal sonography and magnetic resonance (MR) imaging have a low sensitivity for the diagnosis of peritoneal and ovarian implants and adhesions. Transvaginal sonography is useful in the diagnosis of ovarian endometriomata, providing the diameter is larger than 2 cm. As transvaginal sonography costs less than MR imaging, transvaginal sonography might be the preferred method of confirming a sizeable endometrioma. Posterior pelvis endometriosis is an infiltrating adenomyotic lesion with microendometriomata. High-resolution transvaginal ultrasonography, and in particular MR imaging, are increasingly used to diagnose the presence and extent of infiltrating lesions and the involvement of rectosigmoid and ureters.