Hyper-immunoglobulin M (IgM) syndromes are primary immunodeficiencies characterized by normal or elevated serum IgM levels with the absence of other isotypes, pinpointing to a defect in the Ig class switch recombination (CSR). The delineation of hyper-IgM syndromes made it possible to better define the mechanisms underlying the two major events of antibody maturation in humans, CSR and introduction of somatic hypermutation (SHM) in the variable region of immunoglobulins. The description of the activation-induced cytidine deaminase (AID) deficiency, characterized by a defect in both CSR and SHM, demonstrated for the first time that this molecule acts as a master player in the antigen-induced Ig gene-modification events responsible for both CSR and SHM. However, deleterious mutations located in the C-terminus lead to a CSR defect without affecting SHM, providing evidence for a role of AID in CSR distinct from the cytidine deaminase activity, likely by binding to a specific CSR cofactor. Molecular causes of two other hyper-IgM conditions have not yet been defined. However, they may be caused by either a defect in AID targeting on S regions or a CSR-specific DNA-repair defect. The mechanism of action of AID remains somewhat debated, but the observation that uracil-DNA-glycosylase deficiency leads to a severe hyper-IgM syndrome strongly argues in favor of a DNA-editing activity of AID.