Warfarin remains the drug of choice for long-term anticoagulation management in a variety of conditions. Despite an established role in prevention of thromboembolic events such as stroke, warfarin continues to be underutilized because of its association with serious drug-related adverse events. Lacking alternative therapeutic approaches, intensive research in the past decade has focused on making anticoagulation with warfarin safer. Much emphasis has been placed on defining factors associated with the wide individual variability in warfarin dose. Polymorphic sites in three genes, cytochrome P450 (CYP) 2C9, vitamin K 2,3 epoxide reductase complex 1 (VKORC1), and CYP4F2, have been shown to affect stable warfarin dose. An overview of the persistent issues related to warfarin therapy and our current understanding of the genetic and clinical factors affecting warfarin dosing is presented. Finally, unresolved issues in improving clinical care of warfarin patients and future directions are provided.