The present review summarizes key progress made in characterizing the small nucleolar RNAs (snoRNAs) of eukaryotic cells. Recent studies have shown snoRNA populations to be substantially more complex than anticipated initially. Many newly discovered snoRNAs are synthesized by an intron-processing pathway, which provides a potential mechanism for coordinating nuclear RNA synthesis. Several snoRNAs and snoRNP proteins are known to be needed for processing of ribosomal RNA, but precise functions remain to be defined. In principle, snoRNAs could have several roles in ribosome synthesis including: folding of pre-rRNA, formation of rRNP substrates, catalyzing RNA cleavages, base modification, assembly of pre-ribosomal subunits, and export of product rRNP particles.