Pre-mRNA splicing is a critical step in eukaryotic gene expression and regulation. The removal of introns occurs in spliceosome, a macromolecular machine containing small nuclear ribonucleoproteins (snRNP), heterogeneous nuclear ribonucleoproteins (hnRNP) and K-homologous domain (KH) proteins. Crystallographic and NMR studies have begun to provide insight into the architecture of snRNP and the structural basis for RNA-protein as well as protein-protein interactions involved in pre-mRNA splicing. Elucidating molecular mechanisms underlying pre-mRNA splicing awaits systematic structural analyses of splicing factors and complexes.