Drug allergy has become a common and important medical problem. At least 2 to 3 million people have had reactions to penicillin, some of which have been serious. A variety of other drugs, among which are other antibiotics as well as analgesics, sedatives, serums, and organ extracts, have frequently produced allergic symptoms. The effects have ranged from urticaria and other rashes to anaphylaxis, blood changes, periarteritis, and visceral lesions including hepatitis. The allergic state can be diagnosed by skin tests in some cases and by in vitro tests in others, but in most cases the history is most important. A large proportion of the serious reactions are preventable by a knowledge of the special proclivities of the drug, by conservatism, by using appropriate tests, and by alertness during therapy.