Idiopathic anaphylaxis (IA) is a life-threatening allergic disease and the most common diagnosis given to patients following an anaphylactic event. The inability of the healthcare provider and the patient to identify the trigger for anaphylaxis makes standard allergen avoidance measures ineffectual. IA is diagnosed after other causes of anaphylaxis have been excluded. Mast cell activation syndromes (MCAS), mastocytosis, IgE to galactose-alpha-1,3-galactose (α-gal), and certain medications have recently been recognized as causes of anaphylaxis that were previously labeled idiopathic. This review will describe the epidemiology and proposed theories of pathogenesis for IA, its diagnostic approach, its clinical management, and examine newly recognized disorders that were previously labeled as idiopathic anaphylaxis.