Adenosine-to-inosine (A-to-I) RNA editing increases the complexity of the human transcriptome and is essential for maintenance of normal life in mammals. Most A-to-I substitutions occur within repetitive elements in the genome, mainly in Alu repeats. The phenomenon of A-to-I editing is far less abundant in mice, rats, chickens and flies than in humans, which correlates with the relative under-representation of Alu repeats in these non-primate genomes. Here, we review the recent results of bioinformatic and laboratory approaches that have estimated the extent of the editing phenomenon. We discuss the possible biological relevance of the editing pathway, its possible interaction with other cellular pathways that respond to double-stranded RNA and its possible contribution to the accelerated evolution of primates.