The hybridoma technique for obtaining a monoclonal antibody of defined specificity, as first reported by Köhler and Milstein, 1 has an enormous impact on many areas of biological and medical research. Table 1 is a summary of various research areas where m Ab has been applied. The number of publications pertaining to hybridomas is amazingly large and a thorough review of the use of mAb in research is almost impossible. There are already a few review books and many review papers on various applications of the hybridoma technology. Some of these references are included in Table l. 2–10 In this chapter, we would like to make a general review of the use of mAb as research and diagnostic probes using a few examples in each area.