Immunoassay is a biochemical method that measures the presence or concentration of an analyte referred as an antigen in a solution through the use of an antibody or immunoglobulin. Immunoassays are now very widely used in the clinical laboratory, either because no other type of assay system is feasible or because they are often the most effective and suitable of the possible analytical methods. The major trend has been away from liquid-phase assays involving radioisotopic labels, towards fast homogeneous or solid-phase assays capable of operation anywhere; and towards precise and reliable non-isotopic, automated or semi-automated laboratory assays, often with detection limits measured in pico- or attomoles. The use of monoclonal antibodies is now widespread, and the methodologies of labels and of solid-phase components are much more sophisticated. New assay formulations, novel homogeneous systems, immunosensors, free-analyte assays, ultra-sensitive detection systems are being developed. Recent trends has been to enhance the sensitivity of the assays that may also have applications in measurement of other tumor markers, providing earlier detection of cancer or of relapse, Alzheimer's disease and in measurement of certain cardiac markers. More importantly, it is believed that high-sensitivity assays are expected to enable about a 50-fold expansion in the number of protein markers that can be analyzed in the clinical lab. At present, due to the sensitivity limitations of existing immunoassays of about 10-15 molar, only about 2,000 proteins are routinely analyzed by clinical labs. However, about 100,000 proteins are believed to exist in human blood, most of which are not analyzed due to limitations of existing assays. New high-sensitivity tests could potentially drive a significant expansion of the immunoassay market by enabling an entirely new set of markers to be analyzed. The present book is intended to overview different immunoassays including the most recent development trend.