This chapter offers detailed coverage of the history, etiology, pathobiology, epidemiology, diagnosis, and intervention strategies, as well as the economic and public health significance, for infectious bursal disease (IBD). Studies to identify the etiologic agent of IBD were clouded by the presence of infectious bronchitis virus with nephropathogenic tendencies in the kidneys of field cases. IBD is caused by IBD virus (IBDV), a double-stranded RNA birnavirus which is highly resistant to harsh environmental conditions. In the authors' experience based on the experimental reproduction of acute IBD in specific pathogen free white leghorn chickens, “variant” IBDV induces little if any clinical signs and mortality, but marked bursal lesions. Nucleic acid probes and antigen-capture enzyme immunoassays using monoclonal antibodies to detect and differentiate IBDV directly in tissues may prove beneficial for rapid diagnosis and typing of field viruses.