Publisher Summary This chapter discusses the ultrasound guidance for endovenous treatment. Duplex ultrasound is essential in every phase of CVI patient care and has played a pivotal role in the recent revolution in venous therapy. Experience, critical thinking, uniform testing, and insight in the pathology are necessary to achieve satisfactory results. Duplex ultrasound sonography is the best choice for evaluation of venous reflux in the lower extremities. It is inexpensive, noninvasive, and generally acceptable to the patient. It provides direct visualization, localization, and quantitation of venous reflux with a surprisingly high sensitivity and specificity. Duplex ultrasound findings have been confirmed by angioscopic observation of incompetent vein valves in advanced chronic venous insufficiency. It is demonstrated that high peak reflux velocities, reflux duration greater than 3 seconds, and an enlarged valve annulus measured by duplex ultrasonography at the SFJ are all closely related to angioscopically deformed and incompetent terminal valves. The ultrasound examination is conducted with the patient Standing. This position has been found to maximally dilate leg veins and challenges vein valves. Sensitivity and specificity in detecting reflux are increased in examinations performed with the patient standing rather than when the patient is supine. Supine examinations for reflux are unacceptable.