The right and left iliac arteries have been described as the terminal branches of the abdominal aorta (Fig. 19.1). The abdominal aorta bifurcates at the level of the fourth lumbar vertebra, into two large arteries, the right and left common iliac arteries, supplying the pelvis and the lower extremities. The right common iliac artery crosses to the right over the left common iliac vein, very often causing compression of that vein with occlusion or severe stenosis. Compression of the left iliac vein may cause hypertension in the venous circulation of the left lower extremity and deep venous thrombosis, known as iliac compression syndrome, May-Thurner syndrome, or Cockett syndrome. The anatomic abnormality is present in 15% to 20% of the population. The wall of the common iliac vein thickens and progressive occlusion develops. Adhesion of the iliac vein wall is observed in some cases to cause obstruction (Fig. 19.2). The common iliac arteries usually do not have branches above the bifurcation into the external iliac artery and internal iliac artery. Occasional branches of the common iliac arteries are inferior renal polar arteries with an abnormal origin from the common iliac, usually in a situation of multiple renal arteries. Ectopic pelvic kidneys are rare, in about 0.1% of the population, and single or multiple renal arteries may also originate from the iliac arteries, most commonly from the common iliac arteries. The common iliac artery may give small branches to the surrounding tissues, peritoneum psoas muscle, ureter, and pelvic nerves. The common iliac arteries divide into the external iliac artery, which runs parallel to the axis of the common iliac artery, and the internal iliac artery, which is a posteromedial branch.