mRNA editing in plastids (chloroplasts) of higher plants proceeds by cytidine-to-uridine conversion at highly specific sites. Editing sites are recognized by the interplay of cis -acting elements at the RNA level and site-specific trans -acting protein factors that are believed to bind to the cis -elements in a sequence-specific manner. The C-to-U editing enzyme, a presumptive cytidine deaminase acting on polynucleotides, is still unknown. The development of methods for the stable genetic transformation of the plastid genome in higher plants has facilitated the analysis of RNA editing in vivo. Plastid transformation has been extensively used to define the sequence requirements for editing site selection and to address questions about editing site evolution. This chapter describes the basic methods involved in the generation and analysis of plants with transgenic chloroplast genomes and summarizes the applications of plastid transformation in editing research.