The 3' ends of eukaryotic mRNAs are generated by posttranscriptional processing of an extended primary transcript. For almost all RNAs, 3' processing consists of two steps: The mRNA is first cleaved at a particular phosphodiester bond downstream of the coding sequence. The upstream fragment then receives a poly(A) tail of approximately 250 adenylate residues whereas the downstream fragment is degraded. The two partial reactions are coupled so that reaction intermediates are usually undetectable. While 3' processing can be studied as an isolated event in vitro, it appears to be connected to transcription, splicing and transcription termination in vivo.