Conformationally constrained peptide ligands offer an attractive format for the development of therapeutics. They can bind with high affinity and selectivity to protein targets, they are small enough to diffuse into tissues, they can be synthesized chemically, and their degradation products are not toxic. In recent years, a number of novel techniques were innovated to develop peptide-based ligands. One such technique is the generation of bicyclic peptides by phage display. This technique is well established in our laboratory and has led to the isolation of high-affinity antagonist for a range of important therapeutic targets. In a first project I aimed at developing bicyclic peptide ligands of the epidermal growth factor receptor Her2. Aberrant expression of Her2 has been implicated in various malignancies including breast cancer. In the treatment of this cancer type several Her2 specific antibodies were proved to be efficient. Due to their small size, bicyclic peptides could potentially offer advantages over antibodies such as ease of synthesis and conjugation, higher molecule-permass ratios, and better tumor penetration. I panned a large bicyclic peptide phage library against the extra-cellular domain of Her2 and obtained a range of peptide sequences binding to Her2 in a specific manner. After affinity maturation, a bicyclic peptide that bound Her2 with a Kd of 304 nM could be obtained. This peptide ligand offers a valuable starting point for further improvement and the development of high-affinity Her2 binders with potential application for tumor imaging and therapy. A second project was triggered by my observation that it is relatively difficult to generate high-affinity bicyclic peptide ligands to proteins with flat and featureless surfaces such as Her2. The major reason for the limited binding affinity of bicyclic peptides was supposed to lie in the lack of a defined secondary structure in solution and the resulting entropic penalty upon binding. To overcome this problem, I proposed to evolve ligands based on a-helices. Chemically stabilized a-helices, also named stapled peptides, were previously developed by rational design. I proposed to evolve a-helical peptides by phage display wherein the helix, is stabilized in a chemical reaction prior to phage panning. I tested this strategy by affinity maturing an alpha-helical peptide binding to beta-catenin. The project resulted with a 250-fold improved ligand of beta-catenin. In a third project, I developed a novel constrained peptide format that I dubbed “helix-loop” motif. A stabilized alpha-helix was expanded with a peptide loop in order to increase the number of amino acids that can formcontacts with a target. The loop was linked to either the N- or C-terminal end of the helix and via a cysteine residue to the chemical linker stabilizing the alpha-helix. Libraries were created adding randomized loops with different length to either side of the peptide. Ligands with improved affinity were isolated against the cancer target beta-catenin. In summary, I have exploited the existing bicyclic peptide format to evolve bicyclic peptide ligands of Her2. In addition, I have developed a new approach to affinity mature stabilized helical peptides by phage display as well as a new constrained peptide format. This new approach should be applicable for affinity maturation of any stabilized alpha-helix. The new peptide format promises the generation of high affinity ligands to any protein target, including Her2.