AbstractPoly(A) tails have long been known as stable 3′ modifications of eukaryotic mRNAs, added during nuclear pre‐mRNA processing. It is now appreciated that this modification is much more diverse: A whole new family of poly(A) polymerases has been discovered, and poly(A) tails occur as transient destabilizing additions to a wide range of different RNA substrates. We review the field from the perspective of poly(A) tail length. Length control is important because (1) poly(A) tail shortening from a defined starting point acts as a timer of mRNA stability, (2) changes in poly(A) tail length are used for the purpose of translational regulation, and (3) length may be the key feature distinguishing between the stabilizing poly(A) tails of mRNAs and the destabilizing oligo(A) tails of different unstable RNAs. The mechanism of length control during nuclear processing of pre‐mRNAs is relatively well understood and is based on the changes in the processivity of poly(A) polymerase induced by two RNA‐binding proteins. Developmentally regulated poly(A) tail extension also generates defined tails; however, although many of the proteins responsible are known, the reaction is not understood mechanistically. Finally, destabilizing oligoadenylation does not appear to have inherent length control. Rather, average tail length results from the balance between polyadenylation and deadenylation. WIREs RNA 2011 2 348–361 DOI: 10.1002/wrna.56This article is categorized under: RNA Processing > 3' End Processing RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms