Peptidomimetics have found wide application as bioavailable, and often potent mimetics of natural peptides. They form the basis of important classes of enzyme inhibitors, they act as receptor agonists and antagonists, and they have even been used to mimic DNA structure. Recent advances in the use of solid-phase organic synthesis have paved the way for the preparation of libraries of these structures to allow the rapid optimization of theri biological properties and hence therapeutic potential. We are also beginning to gain a greater understanding of the structural features of this class of compounds that influence their ability to permeate membranes, and their rate of clearance and metabolism. This volume brings together many of these critical issues by highlighting recent advances in a number of core peptidomimetic-based research.