The spliceosome is a large RNA-protein complex that catalyses the removal of introns from nuclear pre-mRNA. A wide range of biochemical and genetical studies shows that the spliceosome comprises three major RNA-protein subunits, the U1, U2 and [U4/U6.U5] small nuclear ribonucleoprotein particles (snRNPs), and an additional group of non-snRNP protein splicing factors. Rapid progress is being made in unravelling the interactions which take place between these factors during the splicing reaction. The emerging picture of the spliceosome reveals a highly dynamic structure that assembles on pre-mRNA transcripts in a stepwise pathway and is organised, at least in part, by complex RNA base-pairing interactions between the small nuclear RNAs (snRNAs) and the intron substrate. Many of these interactions can be detected both in mammalian and yeast spliceosomes, suggesting that the basic splicing mechanism is an ancient one that has been highly conserved during evolution.