Mitochondria and chloroplasts were originally acquired by eukaryotic cells through endosymbiotic events and retain their own gene expression machinery. One hallmark of gene regulation in these two organelles is the predominance of posttranscriptional control, which is exerted both at the gene-specific and global levels. This review focuses on their mechanisms of RNA degradation, and therefore mainly on the polyadenylation-stimulated degradation pathway. Overall, mitochondria and chloroplasts have retained the prokaryotic RNA decay system, despite evolution in the number and character of the enzymes involved. However, several significant differences exist, of which the presence of stable poly(A) tails, and the location of PNPase in the intermembrane space in animal mitochondria, are perhaps the most remarkable. The known and predicted proteins taking part in polyadenylation-stimulated degradation pathways are described, both in chloroplasts and four mitochondrial types: plant, yeast, trypanosome, and animal.