The exosome, present in both the nucleus and cytoplasm of all eukaryotic cells, is a complex of 3'-5' exoribonucleases containing at least nine core components. Recently, it has been demonstrated, mainly by analyses in yeast, that the nuclear exosome is essential for rRNA processing and sn(o)RNA biogenesis. Furthermore, it is involved in the degradation of improperly processed mRNAs. The cytoplasmic exosome participates in normal mRNA turnover and in the degradation of inherently instable mRNAs that contain AU-rich elements. Therefore, the exosome plays a key role in RNA metabolism.