The fast turnover of mRNA permits rapid changes in the pattern of gene expression. In procaryotes, many enzymes involved in mRNA degradation have been identified and some of these endo- and exo-ribonucleases are now being intensively studied. Some of the structural features of mRNA that influence decay rates have also recently been defined. Although important components of the decay pathway are still elusive, a coherent and simple model for mRNA decay has emerged in the last few years.