Serological testing and the use of immunochemical techniques are well established in the clinical laboratory and then uses are still rapidly increasing. Serological tests are applied for the diagnosis of infectious diseases, autoimmune disorders, allergies, and malignancies. Yet despite their widespread use, immunoassays are inflicted with a number of significant problems that lead to a substantial variation in reliability and accuracy of currently available tests. In the recent process of establishing procedures and criteria for quality assurance in the clinical laboratory an increased awareness of these problems has arisen among the clinical and reference laboratories as well as among the manufacturers of commercial test kits. The recognition of this situation prompted the US National Committee for Clinical Laboratory Standards (NCCLS) to create specific guidelines that address "the generrc problems of preparation and characterization of antigens and antibodies, testing using these reagents, and understanding the results" These guidelines are made public in the NCCLS document I/LA 18-A Specifications for Immunological Testing for Infectious Diseases; Approved Guideline, issued in December 1994 (1). References to this document will be made in this chapter where appropriate, and particularly, attempts will be made to adhere to the definitions of terms and procedures as they are laid down in the NCCLS consensus document.