Over 50% of all current drugs and perhaps an even higher percentage of drug targets have as their major target receptors/acceptors for which the native ligand/recognition motif is a peptide. Furthermore, most targets for drugs are proteins or conjugated proteins. Peptide hormones and neurotransmitters constitute a very large number of these ligands and almost all of them are important in a wide variety of diseases. Thus robust methods for the design, synthesis, and biological evaluation of peptides, proteins, peptide analogs, and peptidomimetics are needed. In this chapter we briefly discuss the major synthetic, structural, conformational, and topographical considerations that go into peptide design of peptide analogs, derivatives, and mimetics that can serve as drug candidates. We then provide a selected discussion on the design of four different examples of peptide hormone and neurotransmitter ligands, and conclude with a brief discussion of future perspectives in this critically important area of current and future drug research.