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Plant metabolic engineering has the potential to provide for the needs of an expanding population. Environmentally benign biosyntheisis of novel materials and pharmaceutical proteins along with the opportunity to improve the productivity and nutritive value of crop plants has focused considerable effort towards the genetic manipulation of crop species. The most important output traits that could be conferred through biotechnology often require the coordinated expression of several foreign genes. Conservative estimates predict some 3000 proteins are posttranslationally imported into plant plastids. Among them are the enzymes of various metabolic pathways such as those involved in the biosynthesis of the tocopherols (vitamin E) and carotenoids (vitamin A), branched chain and aromatic amino acids, and fatty acids. The ability of the chloroplast to integrate and express foreign sequences as operons makes this site an attractive alternative for genetic manipulations. Multigene engineering, high levels of recombinant protein accumulation and the security of transgene containment due to maternal inheritance of plastid genomes in most crop species are some of the features that contribute to the potential of the chloroplast system. Here we offer an overview of the fundamental characteristics of plastid protein expression and consider some possible candidate genes for the improvement of crop species through metabolic engineering of pathways compartmentalized within plastids [ABSTRACT FROM AUTHOR]