The eukaryotic genome codes for most of its proteins though discontinuous coding sequences called exons, which are separated by noncoding sequences known as introns. Following transcription of a gene, these exons must be spliced precisely, removing the intervening introns, to form meaningful mature messenger RNAs (mRNA) that are transported to the cytoplasm and translated by the ribosomal machinery. To add yet another level of complexity, a process known as alternative splicing exists, whereby a single pre-mRNA can give rise to two or more mature mRNAs depending on the combination of exons spliced together. Alternative splicing of pre-mRNAs is emerging as an important mechanism for gene regulation in many organisms. The classic example of splicing as a regulator of genetic information during a developmental process is sex determination in Drosophila. The now well-characterized cascade of sex-specific alternative splicing events demonstrates nicely how the control of splice site selection during pre-mRNA processing can have a profound effect on the development of an organism. The factors involved in pre-mRNA splicing and alternative splice site selection have been the subject of active study in recent years. Emerging from these studies is a picture of regulation based on protein-protein, protein-RNA, and RNA-RNA interactions. How the interaction of the various splicing constituents is controlled, however, is still poorly understood. One of the mechanisms of regulation that has received attention recently is that of posttranslational phosphorylation. In the following article, we cite the evidence for a role of phosphorylation in constitutive and alternative splicing and discuss some of the recent information on the biochemistry and biology of the enzymes involved.