The translation machinery is generally designated to synthesize polypeptides consisting of 20 proteinogenic (natural) amino acids. Significantly, the ribosomal synthesis is employed in mRNA–template dependent manner, i.e. the sequence of mRNA dictates that of polypeptide according to the genetic code, enabling us to design the peptide sequence by simply preparing the corresponding mRNA or its DNA as the template for transcription. From the synthetic point of view, however, the fact that the useable monomers are limited to 20 proteinogenic amino acids is a serious restriction for the preparation of peptide library with high chemical diversities. In this review, we shall discuss an emerging new technology, referred to as genetic code reprogramming, that allows us to express natural product–like non–standard peptides using a reconstituted cell–free translation system. This technology will open a new avenue for the discovery of a new class of non–standard peptides.