The large majority of classified primary immune deficiency (PID) diseases present in childhood. Yet, most patients with PID are adults, with a large proportion experiencing onset of symptoms beyond their childhood years. Most of these are diagnosed predominantly with antibody defects, but cellular and other disorders are increasingly being identified in older patients as well. Moreover, advances in clinical immunology are allowing pediatric patients, even those with severe disease, to reach adulthood. Because of differences in the physiology and pathophysiology of children and adults, the presentation, diagnosis, and management of a complex chronic disease could differ significantly between these patient populations and therefore require modifications in approach. (C) 2016 American Academy of Allergy, Asthma & Immunology.