Deep vein thrombosis (DVT) is a ubiquitous process that in the acute setting can lead to pulmonary embolism. Chronically, permanent changes that develop within the venous system following an episode of DVT can produce the postphlebitic syndrome, which is associated with pain, swelling, and ulceration. The postphlebitis syndrome can often mimic acute DVT or coexist with it. The clinical evaluation of DVT is ineffective and necessitates a reliable noninvasive diagnostic technique. Compression ultrasound (US) has proved to be the diagnostic method of choice for detection of extremity clot. Femoral and popliteal veins are routinely evaluated for acute clot, but uncertainty exists concerning the need to evaluate the calf veins similarly. US also can be used to diagnose chronic venous changes, which are indicated by the presence of incompetent valves and retrograde blood flow. Upper-extremity venous thrombosis, often induced by indwelling catheters, can also be diagnosed with US.