Deep vein thrombosis (DVT) is the formation of a blood clot (thrombus) within a deep vein, most commonly in the lower extremities (iliac, femoral, and popliteal veins). Virchow's triad describes the three pathophysiological mechanisms: venous stasis (immobility, prolonged travel, heart failure), endothelial injury (surgery, trauma, IV catheterization), and hypercoagulability (inherited thrombophilias such as Factor V Leiden, acquired states such as malignancy, pregnancy, or oral contraceptive use). Thrombus formation begins when stasis allows accumulation of activated clotting factors, while endothelial damage exposes subendothelial collagen and tissue factor, triggering the coagulation cascade. The thrombus propagates proximally, and the greatest danger is embolization to the pulmonary vasculature causing pulmonary embolism (PE), which occurs in approximately 50% of untreated proximal DVTs. The clinician applies the Wells criteria for clinical probability assessment, orders appropriate diagnostic studies including D-dimer and compression ultrasonography, initiates anticoagulation therapy based on risk stratification, and determines the duration of treatment based on provoked versus unprovoked DVT classification and individual bleeding risk.