The capacity of Watson-Crick base-pair complementarity to direct informational transactions basic to gene expression has long been appreciated. Among RNA molecules, it mediates mRNA-tRNA codon-anticodon pairing and the 16S rRNA-mRNA Shine-Dalgarno interaction. More recently, we have come to realize that the role of RNA may transcend that of intermolecular recognition, per se, to include catalysis. Following the tour-de-force studies of the self-splicing Tetrahymena rRNA precursor, the stage is now set for the primary role of RNA to be revealed in nuclear pre-RNA splicing, which is catalyzed by a large ribonucleoprotein (RNP) complex in the cell nucleus, called the spliceosome. The removal of introns from nuclear pre-messenger RNA (pre-mRNA) shares fundamental properties with certain RNA self-splicing reactions. It therefore seems likely that the major catalytic strategies in nuclear pre-mRNA splicing are carried out by the small nuclear RNAs (snRNAs), which are major constituents of the spliceosome.