Abstract: A naive WW domain peptide library derived from a WW domain peptide sequence which has been diversified by changing the amino acid sequence at one or more positions is provided. The naive WW domain peptide library may be derived from a GroupIV WW domain peptide. Methods for making the naive WW domain peptide library and methods for selected a modified WW domain peptide that binds a target ligand using the naive WW domain peptide library are also provided. Also disclosed are modified WW domain peptides that bind desired target ligands, pharmaceutical compositions comprising such peptides, and uses for such peptides. The modified WW domain peptides have altered, improved or different, target ligand binding characteristics to those of the unmodified WW domain peptides from which they are derived.