The diagnosis of retroperitoneal pathology has historically presented a challenge to physicians. The signs and symptoms of retroperitoneal diseases are myriad and often subtle. Cross-sectional imaging techniques have therefore had a major impact on retroperitoneal diagnosis. Computed tomography (CT) is currently the standard imaging technique for evaluating the retroperitoneum. Magnetic resonance (MR) imaging is also well suited for retroperitoneal imaging and has several advantages over CT. MR imaging is the preferred technique in patients with contraindications to iodinated contrast material due to renal dysfunction or allergy. It excels at evaluating the blood vessels, providing a reliable assessment of vessel patency, aneurysm, or dissection. The ability of MR imaging to acquire images without exposure to ionizing radiation facilitates imaging of obstetric and pediatric patients. The ability to acquire images in any plane is a distinct advantage for assessing the extent and origin of retroperitoneal pathology, although this can now be accomplished with spiral CT as well as MR imaging. The spatial resolution of CT is currently superior to MR imaging for most retroperitoneal studies, but this may change with continued technological advances such as improved phased array coil systems (Bidgood WD Jr, Schiebler ML. Retroperitoneum and mesentery. In: Ros PR, Bidgood WD Jr, eds. Abdominal magnetic resonance imaging. St. Louis: Mosby Year Book, 1993:310-47). Finally, MR imaging provides the flexibility to tailor each examination to the specific clinical question. With few exceptions, anything CT can do in the retroperitoneum, MR imaging can do as well or better.