The pace of discovery in primary immunodeficiency continues to accelerate. In particular, lymphocyte defects have been the source of the most impressive expansion in recent years. Novel forms of agammaglobulinemia, class-switch defects, and T-B⁺ severe combined immunodeficiency have been described. Little by little, the genetic heterogeneity of the common variable immunodeficiency and IgA deficiency phenotypes continues to be unraveled as new molecular defects have been reported in these patients as well. The phenotypic spectrum of DiGeorge syndrome has been further developed, along with promising advances in therapy. Defects of nuclear factor κB regulation and Toll-like receptor signaling have been described, along with defects of chemokine receptors and cytoplasmic proteases. Clinically defined immunodeficiencies, such as hyper-IgE syndrome and idiopathic CD4 lymphocytopenia, are also discussed. Finally, significant adverse effects in some patients have tempered initial enthusiasm for gene therapy.