Macromolecular (specifically peptide-based) drugs could potentially be highly effective medicines. However they have a relatively short duration of action and variable therapeutic index. An example of such a peptide is Glucagon-like Peptide I which could potentially be used as a revolutionary drug for diabetes. This is because it stimulates insulin only when the blood glucose level is high thereby reducing the risk of hypoglycemia (a significant disadvantage of using insulin is that an insulin overdose is the single most potent cause of life-threatening hypoglycemia). However it’s short duration of action (half-life of 2 minutes in plasma) precludes its therapeutic use. The author is grateful to Prof. Richard DiMarchi (Standiford H. Cox Professor of Chemistry and the Linda & Jack Gill Chair in Biomolecular Sciences at Indiana University) for valuable suggestions. The foreword for the book has been written by Prof. Jean Martinez, (Legion d'Honneur awarded by the French Republic; Professor of Chemistry and Medicinal Chemistry of the University of Montpellier, France; and Chairman of European Peptide Society, 2002-2010).