Impact of mRNA processing and/or modifications has long been associated with gene expression regulation. Accumulating evidence shows alternative polyadenylation (APA), as an mRNA related process, to emerge as a widespread mechanism in gene expression regulation. Through selecting alternate (proximal or distal) polyadenylation signals on the 3'-UTR of pre-mRNAs, APA generates multiple transcript isoforms which may even create proteomic diversity. Depending on the use of proximal or distal polyadenylation sites, 3'-UTR lengths can vary in a tightly controlled manner in a spatial and temporal mode. Therefore, APA and its deregulation with potential consequences are highly relevant to normal and disease states. In this review, in light of recent findings in the literature, mechanism and types of APA and roles of APA in biological processes (i.e. proliferation, development, differentiation, and transformation) are discussed.