Nearly every eukaryotic mRNA contains a poly(A) tail at its 3' end. Unlike the factors that catalyze other RNA processing reactions, the basal polyadenylation machinery consists entirely of proteins. As with other steps in gene expression, however, this machinery is complex, consisting of multiple separable factors. Two multisubunit factors are required for specification of the poly(A) site and formation of a stable protein-RNA complex. Two additional proteins, as well as poly(A) polymerase, join the complex and are required for cleavage of the pre-mRNA and synthesis of the poly(A) tail, respectively. These and other properties suggest at least a superficial similarity between transcription initiation and polyadenylation.