Peptides are naturally occurring molecules composed of a small number of amino acids. They control many vital functions in the organism, such as blood pressure, pain, memory, appetite, growth, ..., which make them very interesting as potential therapeutics. However, their rapid degradation and their poor oral bioavailability limits their application as drugs. Our team develops methods to make these active peptides more stable, so that their life time in the body is prolonged. Moreover, methods are developed to modify their structure into small organic molecules which can be taken orally. Present targets are opioid peptides for pain therapy, radio labelled peptides for tumour diagnosis and tumour therapy, enzyme inhibitors for the treatment of liver pathology and growth hormone release inhibitory peptides. The strategy which is used to design the new compounds is based on the knowledge of the three dimensional structure of the peptides and of their analogs. This is studied by spectroscopic methods and by computer- assisted molecular modelling. Their preparation requires the use of advanced organic synthesis techniques.