Biphasic anaphylactic reactions have been found to develop in as many as 20% of anaphylactic reactions. The biphasic reaction can be less severe, equally severe, or more severe than the initial reaction, ranging in degree from mild symptoms to fatal reactions. In this review, retrospective and prospective studies as well as case studies and case series are discussed in an attempt to gain insight on the incidence of biphasic reactions, the potential clinical characteristics suggestive of a uniphasic reaction developing into a biphasic reaction, and the recommendations for observation periods after an anaphylactic reaction.