Eukayotic pre-mRNAs are spliced to form mature mRNA. The pre-mRNAs alternative splicing greatly increases the diversity of proteins and the complexity of genes expression. The recognition of splice sites can occur across intron (intron definition) or across exon (exon definition). There are many kinds of alternative splicing patterns, including the choice of alternative splicing sites, the choice of alternative splicing ends, the alternative splicing of mutually exclusive exons, and an intron can be excised from or retained in mRNA. The splice sites choice process is regulated by many kinds of cis-acting and trans-acting elements and is closely related with the basic splicing process. Some of the splicing factors in basic splicing process can regulate the alternative splicing. The alternative splicing is also a co-transcriptional process. Promoters can regulate alternative splicing and produce different mRNA isoforms. Many molecular mechanisms of alternative splicing have been proposed, and it was also found that the RNA editing and trans-splicing could also regulate alternative splicing.