This chapter discusses the applications of monoclonal antibodies in virology. A single monoclonal antibody can provide information on protein “relatedness,” structure, function, synthesis, processing, and cellular or tissue distribution and on the association among molecules. The use of monoclonal antibodies provides valuable insight into the working of the protein both as an enzyme and as a target for the host immune response, evolving in reaction to that response. Monoclonal antibodies find application in two main areas: (1) in the field of rapid diagnosis of virus disease in man, animals, and plants and (2) in the extension of virus taxonomy. Monoclonal antibodies may be used to analyze the role of a protein. This ability to distinguish related proteins can be used to provide a genetic marker in recombination experiments. Monoclonal antibodies can detect low amounts of individual virus proteins within the infected cell. They can, thus, provide information concerning the temporal and spatial separation of protein formation and accumulation, and data on protein modification and processing in the infected cell.