Atrial fibrillation (Af) is the most important cause of cerebral embolisms, and the effects of anticoagulation therapy have been firmly established. The use of warfarization in patients with Af is greater than expected in Japan, but the intensity of anticoagulation has often been insufficient. Antiplatelets, especially aspirin, are prescribed because of a possible preventive effect on embolisms. However, there is no evidence of such an efficacy, and the concomitant use of warfarin and antiplatelets is not good and potentially harmful, at least to the brain. The prescription of warfarin is burdensome for many outpatient clinics, probably because the anticoagulation effects of warfarin fluctuate because of the influence of many factors. Several factors, such as renal function, liver function, and some drug interactions, should be considered when we prescribe new oral anticoagulants (NOACs). NOACs are convenient compared to warfarin, and the lower incidence of intracranial hemorrhage allows anticoagulation treatment to be expanded from patients with Af to patients with lower risks of embolism, such as patients with 1 point on the CHADS(2) score. However, it is still unclear how to use the 3 different NOACs properly.