RNA editing encompasses an important class of co- or posttranscriptional nucleic acid modification that has expanded our understanding of the range of mechanisms that facilitate genetic plasticity. Since the initial description of RNA editing in trypanosome mitochondria, a model of gene regulation has emerged that now encompasses a diverse range of biochemical and genetic mechanisms by which nuclear, mitochondrial, and t-RNA sequences are modified from templated versions encoded in the genome. RNA editing is genetically and biochemically distinct from other RNA modifications such as splicing, capping, and polyadenylation although, as discussed in Section I, these modifications may have relevance to the regulation of certain types of mammalian RNA editing. This review will focus on C to U RNA editing, in particular, the biochemical and genetic mechanisms that regulate this process in mammals. These mechanisms will be examined in the context of the prototype model of C to U RNA editing, namely the posttranscriptional cytidine deamination targeting a single nucleotide in mammalian apolipoproteinB (apoB). Other examples of C to U RNA editing will be discussed and the molecular mechanisms-where known-contrasted with those regulating apoB RNA editing.