Book excerpt: Coronaviruses were recognized as a group of enveloped, RNA viruses in 1968 and accepted by the International Committee on the Taxonomy of Viruses as a separate family, the Coronaviridae, in 1975. By 1978, it had become evident that the coronavirus genomic RNA was infectious (i. e. , positive strand), and by 1983, at least the framework of the coronavirus replication strategy had been per ceived. Subsequently, with the application of recombinant DNA techniques, there have been remarkable advances in our understanding of the molecular biology of coronaviruses, and a mass of structural data concerning coronavirus genomes, mRNAs, and pro teins now exists. More recently, attention has been focused on the role of essential and accessory gene products in the coronavirus replication cyde and a molecular analysis of the structure-function relation ships of coronavirus proteins. Nevertheless, there are still large gaps in our knowledge, for instance, in areas such as the genesis of coronavirus subgenomic mRNAs or the function of the coronavirus RNA-dependent RNA polymerase. The diseases caused by coronaviruses have been known for much longer than the agents themselves. Possibly the first coronavirus-related disease to be recorded was feline infectious peritonitis, as early as 1912. The diseases associ ated with infectious bronchitis virus, transmissible gastroenteritis virus, and murine hepatitis virus were all well known before 1950.