Since January 2020, Elsevier has established a COVID-19 resource center on its public news and information website, Elsevier Connect, providing free information in English and Mandarin on the novel coronavirus. The company has granted permission to make all its COVID-19-related research available in PubMed Central and other publicly funded repositories, including the WHO COVID database, with unrestricted access for research reuse and analysis, provided that the original source is acknowledged. This permission is granted for as long as the resource center remains active. The review focuses on the Nidovirales order of animal RNA viruses, which includes coronaviruses, toroviruses, roniviruses, and arteriviruses. These viruses possess the largest known RNA genomes (26-32 kb) and are compared with arteriviruses, which have smaller genomes (13-16 kb). The review outlines common and unique features of these viruses, including their genetic plan, genome diversity, replicase machinery, virus particles, and accessory genes. The replicase gene expression is controlled by ribosomal frameshifting and proteases, and a nested set of subgenomic RNAs is synthesized to express conserved structural proteins and accessory proteins. The acquisition of specific enzymes may have improved the fidelity of RNA replication, allowing for genome expansion and the evolution of small and large nidoviruses.Since January 2020, Elsevier has established a COVID-19 resource center on its public news and information website, Elsevier Connect, providing free information in English and Mandarin on the novel coronavirus. The company has granted permission to make all its COVID-19-related research available in PubMed Central and other publicly funded repositories, including the WHO COVID database, with unrestricted access for research reuse and analysis, provided that the original source is acknowledged. This permission is granted for as long as the resource center remains active. The review focuses on the Nidovirales order of animal RNA viruses, which includes coronaviruses, toroviruses, roniviruses, and arteriviruses. These viruses possess the largest known RNA genomes (26-32 kb) and are compared with arteriviruses, which have smaller genomes (13-16 kb). The review outlines common and unique features of these viruses, including their genetic plan, genome diversity, replicase machinery, virus particles, and accessory genes. The replicase gene expression is controlled by ribosomal frameshifting and proteases, and a nested set of subgenomic RNAs is synthesized to express conserved structural proteins and accessory proteins. The acquisition of specific enzymes may have improved the fidelity of RNA replication, allowing for genome expansion and the evolution of small and large nidoviruses.