Abstract: A method is disclosed for identifying a member of a peptide library that interacts with a target molecule in situ, the method including expressing immobilised nucleic acid molecules to produce the peptide library in a way that each member of the peptide library is immobilised on the nucleic acid molecule from which it was expressed; contacting the immobilised peptide library with the target molecule; and detecting an interaction between at least one member of the peptide library and the target molecule. The method further comprises sequencing the plurality of nucleic acid molecules in situ on the solid support, such that the at least one member of the peptide library that interacts with the target molecule can be immediately identified, at least by the sequence of the nucleic acid molecule from which it was expressed, without requiring additional or secondary analysis or characterising procedures in order to identify the useful members of the library.