Over the past 2 decades venous ultrasonography has become the standard primary imaging technique for the initial evaluation of patients for whom there is clinical suspicion of deep venous thrombosis (DVT) of the lower extremity veins. This article addresses the role of duplex ultrasonography and color Doppler ultrasonography in today's clinical practice for the evaluation of patients suspected of harboring a thrombus in their lower extremity veins. It reviews the clinical presentation and differential diagnoses, technique, and diagnostic criteria for acute and chronic DVT. In addition, it addresses the sonographic evaluation of venous insufficiency.