X-linked agammaglobulinemia (XLA) is the prototypic humoral immunodeficiency arising from a defect in B-cell maturation and the consequent failure of immunoglobulin production. The defective gene in XLA has now been identified and termed BTK. In this article, the clinical manifestations of XLA and the role of Btk protein in intracellular signaling and its interactions are discussed. This discussion leads to an increased understanding of the molecular mechanisms in the development of B cells and has allowed the authors to develop more accurate methods for diagnosis of humoral abnormalities.