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UFH enhances antithrombin-mediated inactivation of thrombin (factor IIa) and factor Xa, as well as other clotting factors. LMWHs, including enoxaparin, dalteparin, and tinzaparin, act preferentially against factor Xa; fondaparinux selectively inhibits factor Xa through antithrombin and has no factor IIa effect. The agents differ in route, onset, half-life, clearance, and monitoring needs. Their clinical uses include venous thromboembolism prevention and treatment, acute coronary syndromes, percutaneous coronary intervention, bridging, and dialysis circuit anticoagulation. Safe use also requires attention to bleeding, heparin-induced thrombocytopenia, reversal options, contraindications, renal function, pregnancy, and interactions that can increase bleeding or alter UFH effect.