Maturation of pre-ribosomal RNA (pre-rRNA) in eukaryotic cells takes place in the nucleolus and involves a large number of cleavage events, which frequently follow alternative pathways. In addition, rRNAs are extensively modified, with the methylation of the 2'-hydroxyl group of sugar residues and conversion of uridines to pseudouridines being the most frequent modifications. Both cleavage and modification reactions of pre-rRNAs are assisted by a variety of small nucleolar RNAs (snoRNAs), which function in the form of ribonucleoprotein particles (snoRNPs). The majority of snoRNAs acts as guides directing site-specific 2'-O-ribose methylation or pseudouridine formation. Over one hundred RNAs of this type have been identified to date in vertebrates and the yeast Saccharomyces cerevisiae. This number is readily explained by the findings that one snoRNA acts as a guide usually for one or at most two modifications, and human rRNAs contain 91 pseudouridines and 106 2'-O-methyl residues. In this article we review information about the biogenesis, structure and function of guide snoRNAs.