We report the updated classification of primary immunodeficiencies (PIDs) compiled by the Expert Committee of the International Union of Immunological Societies. In comparison to the previous version, more than 30 new gene defects are reported in this updated version. In addition, we have added a table of acquired defects that are phenocopies of PIDs. For each disorder, the key clinical and laboratory features are provided. This classification is the most up-to-date catalog of all known PIDs and acts as a current reference of the knowledge of these conditions and is an important aid for the molecular diagnosis of patients with these rare diseases.