With a few rare exceptions, genes encoding pentatricopeptide (PPR) proteins are present in all sequenced eukaryotic genomes but absent from prokaryotic and archaeal genomes. The family has greatly expanded in plants, to more than 400 genes in each species. So far, the evidence indicates that PPR proteins are generally involved in regulation of organelle genome expression, in other words they are eukaryotic proteins selected for the control of genomes of prokaryotic origin. PPR proteins are localised in both plastids and mitochondria, and appear to have similar roles in both cases. They have been implicated in almost all stages of gene expression, including messenger RNA (mRNA) transcription, splicing, processing, editing, translation and stability. The most probable hypothesis for explaining these diverse roles is that PPR proteins are sequence-specific RNA-binding adaptors capable of directing effector enzymes to defined sites on mRNAs. Much of the recent interest in the role of PPR proteins in mitochondria has been driven by the discovery that most cytoplasmic male sterility systems comprise fertility restorer genes that are members of this fascinating family.