Publisher Summary The basic recognition sites (5 ‘ , 3‘, and branchpoint) for intron splicing in plants do not differ significantly from those of vertebrates. It is also demonstrated, by the mutations of splice-site recognition sequences, that the 5 ‘ GT or 3‘ AG dinucleotides are essential for proper splicing in monocots. The differences in base composition observed in monocot and dicot introns are described in this chapter. These differences could explain some of the reports of inefficient splicing or nonsplicing observed in dicot cells with genes containing monocot introns. The reports of splicing difficulties between vertebrate and plant introns in heterologous systems may be attributed to the lack of a good polypyrimidine sequence adjacent to the 3‘ splice-site in the plant introns. Moreover, previous reports by other investigators have broadly generalized the observations of inefficient or incorrect splicing in dicot cells (with vertebrate introns) to be indicative of results in all plants. The chapter also presents experimental data indicating that (1) splicing is required to achieve enhanced expression of reporter genes in protoplasts and (2) the various introns are spliced with differing efficiencies. These data also demonstrates that the degree of enhanced β- glucuronidase (GUS) expression obtained in assays is directly correlated with the extent of RNA splicing. Utilizing the transient GUS assay system as a measure of splicing efficiency, it has been found that maize cells are very proficient at processing introns from both monocot and dicot sources, whereas tobacco or carrot cells are not.