Abstract: Genes producing functional RNAs rather than protein products form a large and variegated class in all genomes, from bacteria to mammals. In higher organisms. non-coding RNA (ncRNA) appears to dominate the whole genomic output, and is not surprising that the range of known RNA-induced phenomena is rapidly expanding. The central importance of RNA signaling to eukaryotic cell has become apparent in the last few years, when a large bulk of evidence has pointed out novel roles for ncRNA molecules in both genetic and epigenetic processes. The family of nc-RNA genes comprises many small nucleolar RNAs (snoRNAs) that guide the maturation or post-transcriptional modification of target RNA molecules. Most snoRNAs fall into two classes called box C/D and box H/ACA snoRNAs, with each class defined by the presence of common sequence motifs and common associated proteins. A few snoRNAs in either class are required for definite pre-rRNA cleavages and essential for viability, whereas most are responsible for the 2’-O-ribose methylation (C/D) or pseudouridylation (H/ACA) of target RNA molecules respectively. The C/D class guides site-specific 2’-Oribose methylation by base-pairing of the 10-21 nt-long sequence positioned upstream from a D (or an internal D’) box to the target RNA, with the nucleotide positioned 5 base pairs (bp) upstream from the D/D’ box selected for methylation. Although most of the C/D and H/ACA box snoRNAs are involved in modifications of ribosomal RNA (rRNA), other types of RNA molecules, as tRNAs, snRNAs, and possibly mRNAs, might be recognised as targets. Despite the importance of their functional roles, most of snoRNAs have not yet been identified even in organisms whose genome has been completely sequenced.