The diagnosis of deep vein thrombosis (DVT) is discussed. Accurately diagnosing DVT is critical to making appropriate treatment decisions. Careful patient assessment, combined with objective testing, improves the accuracy of the diagnosis and reduces the likelihood of inappropriate treatment. Venography remains the reference standard for the diagnosis of DVT but is expensive, invasive, and prone to inducing complications. Ultrasonography has become the most frequently used noninvasive test for symptomatic DVT because it is highly sensitive and specific in the hands of an experienced examiner. Impedance plethysmography also has been widely used, but recent studies suggest that it is less sensitive than once believed. The radiolabeled 125I-fibrinogen uptake test is no longer available because of concerns about the transmission of blood-borne pathogens. Current thermographic techniques have relatively high sensitivity but poor specificity for DVT. Magnetic resonance imaging and computed tomography are useful adjunctive tests, but their use is limited by cost and availability. D-dimer whole-blood testing may prove to be a rapid and convenient means of ruling out the diagnosis of DVT at the bedside, but further study is needed. When used alone, none of the noninvasive methods is sufficiently sensitive for the evaluation of asymptomatic patients. The diagnostic strategy used should be based on whether the patient is symptomatic or asymptomatic, whether the event is a first one or is recurrent, and a careful clinical assessment. Accurate diagnosis of deep vein thrombosis relies on both testing and patients assessment.