A polyA (pA) tail is an essential modification added to the 3' ends of a wide range of RNAs at different stages of their metabolism. Here, we describe the main sources of polyadenylation and outline their underlying biochemical interactions within the nuclei of budding yeast  Saccharomyces cerevisiae , human cells and, when relevant, the fission yeast  Schizosaccharomyces pombe  Polyadenylation mediated by the  S. cerevisiae  Trf4/5 enzymes, and their human homologues PAPD5/7, typically leads to the 3'-end trimming or complete decay of non-coding RNAs. By contrast, the primary function of canonical pA polymerases (PAPs) is to produce stable and nuclear export-competent mRNAs. However, this dichotomy is becoming increasingly blurred, at least in  S. pombe  and human cells, where polyadenylation mediated by canonical PAPs may also result in transcript decay.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.