This review describes the clinical, immunologic and pathologic features of two naturally-occurring models of severe combined immunodeficiency (SCID) in domestic animals that represent different forms of human SCID. Canine X-linked SCID (XSCID) has an X-linked recessive mode of inheritance and, as such, represents a model for the most common form of human SCID in the United States. Affected dogs have normal percentages of circulating B cells and low to normal percentages of phenotypically mature, but nonfunctional T cells. Severe combined immunodeficiency in the horse is an autosomal recessive form of SCID that is characterized by a profound lymphopenia affecting both the B and T cell lineage most likely due to a lymphoid stem cell defect. Since these diseases are naturally-occurring in an outbred species, like man, they represent unique animal models of their respective human counterparts in which to determine the underlying immunologic defect(s), to evaluate novel approaches to immunotherapy or gene therapy, and to evaluate therapeutic regimens for opportunistic infections associated with SCID.