Deficiency in the enzyme adenosine deaminase (ADA) in humans manifests primarily as severe lymphopenia and immunodeficiency, resulting in death by 6 months of age, if untreated. In this review, we discuss phenotypical, biochemical, and metabolic hallmarks of the disease, and describe a mouse model in which levels of ADA can be biochemically and genetically manipulated. This model provides exciting possibilities for uncovering the mechanisms by which this purine catabolic enzyme affects lymphopoiesis.