Almost all messenger RNAs carry a polyadenylate tail that is added in a post-transcriptional reaction. In the nuclei of animal cells, the 3'-end of the RNA is formed by endonucleolytic cleavage of the primary transcript at the site of poly(A) addition, followed by the polymerisation of the tail. The reaction depends on specific RNA sequences upstream as well as downstream of the polyadenylation site. Cleavage and polyadenylation can be uncoupled in vitro. Polyadenylation is carried out by poly(A) polymerase with the aid of a specificity factor that binds the polyadenylation signal AAUAAA. Several additional factors are required for the initial cleavage. A newly discovered poly(A)-binding protein stimulates poly(A) tail synthesis and may be involved in the control of tail length. Polyadenylation reactions different from this scheme, either in other organisms or under special physiological circumstances, are discussed.