Small nucleolar RNAs (snoRNAs) are one of the most numerous and well-studied groups of non-protein-coding RNAs. In complex with proteins, snoRNAs perform the two most common nucleotide modifications in rRNA: 2'-O-methylation of ribose and pseudouridylation. Although the modification mechanisms and shoRNA structures are highly conserved, the snoRNA genes are surprisingly diverse in organization. In addition to genes transcribed independently, there are genes that are in introns of other genes, form clusters transcribed from a common promoter, or cluster in introns. Interestingly. one type of gene organization usually prevails in different taxa. Vertebrate snoRNAs mostly originate from introns of protein-coding genes; a small group of snoRNAs are encoded by introns of genes for noncoding RNAs.