Adenosine deaminase (ADA) deficiency leads to an accumulation of toxic purine degradation by-products, most potently affecting lymphocytes, leading to adenosine deaminase-deficient severe combined immunodeficiency. Whilst most notable affects are on lymphocytes, other manifestations include skeletal abnormalities, neurodevelopmental affects and pulmonary manifestations associated with pulmonary-alveolar proteinosis. Affected patients present in early infancy, usually with persistent infection, or with pulmonary insufficiency. Three treatment options are currently available. Initial treatment with enzyme replacement therapy may alleviate acute symptoms and enable partial immunological reconstitution, but treatment is life-long, immune reconstitution is incomplete, and the reconstituted immune system may nullify the effects of the enzyme replacement. Hematopoietic stem cell transplant has long been established as the treatment of choice, particularly where a matched sibling or well matched unrelated donor is available. More recently, the use of gene addition techniques to correct the genetic defect in autologous haematopoietic stem cells treatment has demonstrated immunological and clinical efficacy. This article reviews the biology, clinical presentation, diagnosis and treatment of ADA-deficiency.