This chapter contains sections titled: Introduction Consequences of Adenosine Deaminase Deficiency Physical-Chemical Properties of Adenosine Deaminase and Tissue Distribution The Role of Adenosine Deaminase in the Purine Salvage Pathway Cell Culture Models for Studying Adenosine Deaminase Deficiency Biochemical Mechanisms of the Immunodeficiency Caused by Adenosine Deaminase Deficiency Therapy Adenosine Deaminase Deficiency and Immunocompetency Significance to Biomedical Sciences