FeHne infectious peritonitis (FIP) is one of the most intriguing diseases caused by a coronavirus. It involves immune-media ted phenomena such as antibodydependent enhancement of virus infection-and immune complex-induced pathology. Furthermore, there is increasing evidence for the existence of a carrier state. FIP and feHne coronaviruses have been extensively reviewed (Pedersen, 1976b(Pedersen, , 1983a Barlough and Stoddart, 1986, 1990j Olsen, 1993. Most of these reviews emphasized the pathology, epidemiology, and classical virology. During the last 10 years, our knowledge of the molecular biology of feHne coronaviruses has increased considerably. We will summarize clinicopathological findings and the his tory of FIP research only briefly. The focus will be on the molecular aspects of feHne coronaviruses and of FIP pathogenesis.