Assays performed in the diagnostic immunology laboratory support the diagnosis and management of a wide spectrum of clinical conditions. This chapter reviews immunologic principles as they apply to diagnostic laboratory assays. Most of the determinations are based on well-established principles of antigenantibody reactions. Some of the specific areas discussed include flow cytometric analyses, critical in the care of patients with hematologic malignancies, with the acquired immunodeficiency syndrome, or undergoing transplantation, and protein electrophoretic assays to identify the presence of monoclonal gammopathies. We also discuss the use of molecular techniques in the diagnosis of hematologic malignancies and primary immunodeficiencies, characterization of the major histocompatibility complex, and enumeration of viral burden.