Duplex sonography for the examination of the arteries of the lower extremity is performed with 5–7.5 or 10 MHz transducers. The color-duplex ultrasound combines the standard duplex technology with a real-time color display of blood flow. Color duplex is readily available, noninvasive, cost-effective, and highly accurate. It can easily differentiate between true aneurysms and pseudoaneurysms. Color-duplex ultrasound imaging for the diagnosis of femoral and popliteal aneurysms should include the measurement of the maximum diameter of the aneurysm, proximal and distal extent of the aneurysm, and the presence of intraluminal thrombus. No flow is detected in a patient with acute thrombosis of the popliteal aneurysm, resulting in acute ischemia of the lower extremity.