Common variable immunodeficiency (CVID) is a primary immunodeficiency that is characterized by low level of serum immunoglobulins and an increased susceptibility to infections. The patients show a variety of clinical, cellular, and immunological defects, may develop autoimmune disease, and are susceptible to malignancy. The recent identification of four genetic defects that result in the CVID phenotype demonstrates that the genetic basis of CVID is highly variable. The responsible gene products include, ICOS, TACI, BAFF-R, CD19. Insufficiency of each molecule disrupts B cell maturation, function and differentiation at a different stage. Despite the elucidation of responsible genes, the molecular mechanisms leading to the immune defects are still yet to be understood. In this paper, we overview the molecular basis of CVID, and will provide some data on how the defect leads to autoimmunity.