I present an overview of HLA typing, a method of evaluating donor-recipient compatibility. The major histocompatibility system in man, the HLA region, considered to control the immunologic response to transplantation and perhaps to certain diseases, is discussed in relation to chromosomal location, chromosomal mapping, association with other markers, nomenclature of the gene products (the HLA antigens), inheritance of the HLA antigens, and methods of determining these antigens. Use of HLA typing is currently most applicable in the correlation of certain diseases with HLA antigen frequency, in evaluating donor-recipient pairs for renal and bone marrow transplantation and for transfusion of platelets and granulocytes and in paternity testing.