Pre-messenger RNA (pre-mRNA) splicing or the removal of introns from pre-mRNA transcripts is a key process in the maturation of mRNA. This process requires the assembly of a large complex of RNA and protein molecules, called the splicosome, on the pre-mRNA transcripts. Molecular and biochemical analyses of plant intron sequence and structure and of the components of the plant spliceosome are providing information on the mechanism of intron recognition and splice site selection in both monocoty-ledonous and dicotyledonous plants. This knowledge will help in gaining an understanding of phenomena such as the difference in splicing between monocotyledonous and dictoyledonous plants, the enhancement of gene expression brought about by the presence of some introns and alternative splicing. The importance of introns and pre-mRNA splicing to accurate and regulated gene expression, therefore, is of direct relevance to transgene expression and genetic manipulation.