Allergic reactions occuring under anesthesia constitute one of the most important complications. They occur mainly due to injection of multiple drugs at the same time in large concentrations. Example of these drugs are intravenous anesthetics, inhaled anesthetics, muscle relaxants, opioids, local anesthetics and antibiotics. Allergic reactions occur also with blood products transfusion and usage of plasma expanders. Understanding etiology, pathophysiology, prophylaxis and management of these reactions is a must for the anesthetists.