Cyclic peptides (or cyclic proteins) are polypeptide chains wherein the amino termini and carboxyl termini, amino termini and side chain, carboxyl termini and side chain, or side chain and side chain are linked with a covalent bond that generates the ring. A number of cyclic peptides have been discovered in nature and a plethora have been synthesized in the laboratory. Their length ranges from just two amino acid residues to hundreds. These cyclic peptides have several applications in medicine and biology.