Hereditary angioedema (HAE) is defined as a rare genetic disease with recurrent episodes of localized bradykinin-mediated swelling of the deep tissues of the skin, respiratory, and gastrointestinal tracts that can be life threatening. Classification of HAE has evolved over time with our further understanding of clinical phenotypes, underlying causes, and available testing. In most cases, HAE is caused by a deficiency of C1-esterase inhibitor (C1-INH) on the Serpin Family G Member 1 (SERPING1) gene, either through decreased amounts of C1-INH protein (C1-INH-HAE, type 1) or decreased function of C1-INH (C1-INH-HAE, type 2). HAE with normal C1-INH levels and function are divided into unknown cause or into non-C1-INH-HAE forms, which include known mutational defects in factor XII (called FXII-HAE in the Hereditary Angioedema International Working Group consensus), angiopoietin-1, plasminogen, and kininogen 1 genes. It is possible that, after an initial workup, a patient without a family history of HAE could be classified with an acquired form of angioedema (nonhereditary) that may later prove to be HAE due to a de-novo SERPING1 mutation. Because there are forms of nonhistaminergic (H1-antihistamine unresponsive) angioedema that appear clinically very similar to HAE, it is essential that the patient undergoes a thorough clinical history and diagnostic evaluation to ensure that he or she is properly diagnosed and classified.