Recent genetic and biochemical experiments have revealed an intimate and dynamic role for small nuclear RNAs (snRNAs) in multiple steps of RNA-splicing reactions. Both snRNA-substrate and snRNA-snRNA interactions are involved. These interactions concern not only splice site and branch point definition, but also the catalytic reactions of the first and second steps of splicing. Studies reveal a striking conservation between snRNA interactions and interactions found in RNAs encoded by genes with group II self-splicing introns.