Coronaviruses are a group of enveloped viruses with a nonsegmented, positive-sensed RNA genome. These viruses have common morphology with characteristic petal-shaped spikes, and were grouped together as Coronaviridae in 1974. Coronaviruses have been isolated from many animal species. Among these are several economically and clinically important viruses which infect various species, causing a variety of respiratory and gastrointestinal illnesses. Coronaviruses contain the largest RNA genome among all of the RNA viruses. In most of the coronavirus-infected cells, the relative rate of synthesis of all of the mRNA species is constant throughout the infection. The RNA replication of coronaviruses clearly utilizes very unique mechanisms. These mechanisms include leader-primed RNA transcription and discontinuous, nonprocessive RNA replication. Consideration of the RNA replication scheme of coronaviruses suggests that at least three or four different RNA polymerases, or three or four different modified forms of the same polymerase, are required for the complete replication cycle of coronaviruses.