Splicing of the precursors of eukaryotic mRNA and some non-coding RNAs is catalyzed by the 'spliceosome', which comprises five RNA-protein complexes (small nuclear ribonucleoproteins, or snRNPs) that assemble in an ordered manner onto precursor-mRNAs. Much progress has been made in determining the gross morphology of spliceosomal assembly intermediates. Recently, the first crystal structure of a spliceosomal snRNP has provided significant insight into assembly and architecture of spliceosomal snRNPs in general and the structure-function relationship of human U1 snRNP in particular.