In patients with ALPS, defective homeostasis of lymphocytes is reflected in abnormal accumulation of lymphocytes, leading to lymphadenopathy, (hepato)splenomegaly and hypersplenism, autoimmunity due to a failure to remove autoreactive lymphocytes, and inappropriate survival of lymphocytes associated with an increased occurrence of lymphoma. Several of the laboratory findings are unique for ALPS and reflect defective Fas-mediated apoptosis and abnormal immune regulation. Much has been learned about the molecular mechanisms that underlie defective Fas-mediated apoptosis and the complex relationship between genotype, phenotype and disease penetrance. Family studies strongly suggest the contribution of one or more additional factors to the pathogenesis of ALPS. This may pertain to defective immunoregulation by an altered IL-2 / IL-2 receptor system, reflected in the specific loss of CD4+ / CD25+ T cells, and / or by the highly increased IL-10 levels, but other factors may equally be involved. Treatment strategies remain mostly targeted at the disease manifestations, but more specific therapies directed at the primary pathogenic defects themselves might become possible in the future. Continued efforts directed at both careful clinical follow-up and basic scientific investigation are needed to increase our understanding of the incidence, natural history, and pathogenesis of ALPS. In return, this may prove of benefit for the understanding of autoimmune disease in general.