The transfusion of whole blood and blood components is usually a temporarily effective means of correcting red cell, white cell, platelet, and coagulation factor deficits. Unfortunately, blood and blood components are occasionally unavoidably unsafe, which results in a spectrum of adverse reactions following transfusion. These reactions may be immunologically or nonimmunologically mediated, and may vary in severity from mild to fatal. Physicians should weigh the potential risks versus the potential benefits of each blood transfusion. Only when the benefits clearly outweigh the risks should a blood transfusion be administered. Transfusionists must carefully monitor each blood transfusion for signs and symptoms of transfusion reactions in order to minimize their adverse effects.