Abstract: Described are aminoacyl tRNA analogues which comprise a tRNA, and an amino acid which acts as an acceptor and donor substrate for ribosome-directed translation, thus, incorporating unusual monomers into non-standard polymers by the action of ribosomes. Also described are methods for producing such tRNA analogues; non-standard polymers; libraries of encoded polymers; methods of screening the libraries; and target members and their uses. A key advantage of synthesizing non-standard polymer libraries of the present invention with aminoacyl tRNA analogues is that large libraries of high complexity can be easily made and functional library members (e.g. novel drugs) readily identified.