Since January 2020, Elsevier has created a free COVID-19 resource center with English and Mandarin information on the novel coronavirus. The center is hosted on Elsevier Connect, a public news and information website. Elsevier grants permission to make all its COVID-19-related research immediately available in PubMed Central and other public repositories for unrestricted research use. The article discusses the Nidovirales order, which includes coronaviruses, toroviruses, and nonviruses with the largest RNA genomes (26–32 kb), called 'large' nidoviruses. These are compared with arteriviruses (13–16 kb), also in the Nidovirales order. Common and unique features of large and all nidoviruses are outlined, including their genetic plan, genome diversity, replicase machinery, virus-specific accessory genes, RNA and protein synthesis mechanisms, and the origin and evolution of nidoviruses with small and large genomes. Nidoviruses use single-stranded, polycistronic RNA genomes that direct the synthesis of replicase subunits, including RNA-dependent RNA polymerase and helicase. Replicase gene expression is controlled by ribosomal frameshifting signals and proteases. A nested set of subgenomic RNAs is synthesized to express 3'-proximal ORFs that encode conserved structural proteins and accessory proteins in some large nidoviruses. The replicase machinery includes RNA-processing enzymes unique to nidoviruses, which may have improved RNA replication fidelity to allow genome expansion. The article also discusses the genome diversity and common genetic plan of nidoviruses, including their replicase machinery, which is conserved across the order. The replicase machinery includes domains such as the 3C-like protease (3CLpro), which is essential for processing replicase polyproteins. The 3CLpro has a chymotrypsin-like fold and is responsible for cleaving the C-terminal half of pp1a and the ORF1b-encoded part of pp1ab. The 3CLpro has evolved into different catalytic centers in different nidovirus branches. The replicase machinery also includes the RNA-dependent RNA polymerase (RdRp), which is essential for RNA replication. The RdRp is the largest conserved domain in nidoviruses and is involved in RNA synthesis. The article also discusses group-specific domains, such as the 3'-to-5' exoribonuclease (ExoN) and ribose-2'-O-methyltransferase (O-MT), which are conserved in large nidoviruses. These domains are essential for viral RNA synthesis and the production of virus progeny. The article concludes by discussing the family-specific virion morphology and major virion components, including the spike (S) and membrane (M) proteins inSince January 2020, Elsevier has created a free COVID-19 resource center with English and Mandarin information on the novel coronavirus. The center is hosted on Elsevier Connect, a public news and information website. Elsevier grants permission to make all its COVID-19-related research immediately available in PubMed Central and other public repositories for unrestricted research use. The article discusses the Nidovirales order, which includes coronaviruses, toroviruses, and nonviruses with the largest RNA genomes (26–32 kb), called 'large' nidoviruses. These are compared with arteriviruses (13–16 kb), also in the Nidovirales order. Common and unique features of large and all nidoviruses are outlined, including their genetic plan, genome diversity, replicase machinery, virus-specific accessory genes, RNA and protein synthesis mechanisms, and the origin and evolution of nidoviruses with small and large genomes. Nidoviruses use single-stranded, polycistronic RNA genomes that direct the synthesis of replicase subunits, including RNA-dependent RNA polymerase and helicase. Replicase gene expression is controlled by ribosomal frameshifting signals and proteases. A nested set of subgenomic RNAs is synthesized to express 3'-proximal ORFs that encode conserved structural proteins and accessory proteins in some large nidoviruses. The replicase machinery includes RNA-processing enzymes unique to nidoviruses, which may have improved RNA replication fidelity to allow genome expansion. The article also discusses the genome diversity and common genetic plan of nidoviruses, including their replicase machinery, which is conserved across the order. The replicase machinery includes domains such as the 3C-like protease (3CLpro), which is essential for processing replicase polyproteins. The 3CLpro has a chymotrypsin-like fold and is responsible for cleaving the C-terminal half of pp1a and the ORF1b-encoded part of pp1ab. The 3CLpro has evolved into different catalytic centers in different nidovirus branches. The replicase machinery also includes the RNA-dependent RNA polymerase (RdRp), which is essential for RNA replication. The RdRp is the largest conserved domain in nidoviruses and is involved in RNA synthesis. The article also discusses group-specific domains, such as the 3'-to-5' exoribonuclease (ExoN) and ribose-2'-O-methyltransferase (O-MT), which are conserved in large nidoviruses. These domains are essential for viral RNA synthesis and the production of virus progeny. The article concludes by discussing the family-specific virion morphology and major virion components, including the spike (S) and membrane (M) proteins in