Aberrant expression of the epidermal growth factor receptor Her2 has been implicated in various malignancies including breast cancer. Monoclonal antibodies and an antibody drug conjugate targeting Her2 have found wide clinical application. Herein, we aimed at developing Her2-specifc ligands based on peptides that have a 100-fold smaller molecular weight than antibodies. Such peptides could potentially offer advantages in the development of ligand drug conjugates, such as ease of synthesis and conjugation, higher molecule-per-mass ratios, and better tumor penetration. Panning of large bicyclic peptide phage display libraries against Her2 yielded a range of Her2-specific ligands having different formats and binding motifs. Strong sequence similarities among several of the isolated peptides indicated that they interact with Her2 in a specific manner. The best bicyclic peptide obtained after affinity maturation bound Her2 with a K-D of 304 nM. The diverse peptide ligands may offer valuable starting points for the development of high-affinity Her2 binders with potential application for tumor imaging and therapy. (C) 2014 Elsevier Ltd. All rights reserved.