Well-managed warfarin therapy remains an important method of anticoagulation in the 21st century, despite the introduction of new antithrombotics into the clinical practice. The main advantages of warfarin are decades of treatment experience, the possibility to monitor its anticoagulant effect using the INR and the last, but not least, the low cost. Currently, approximately 75 % of anticoagulated patients in the Czech Republic are treated with warfarin and warfarin remains the only option for oral anticoagulant therapy in certain clinical conditions (particularly in patients with valvular atrial fibrillation or mechanical heart valves). For physicians across specialties it is still indispensable to master the basics of safe and effective warfarin therapy, including the management of treatment complications.Key words: anticoagulant therapy - INR - thrombosis - warfarin.