Increasing conventional antibiotic resistance has created an urgent need to develop novel therapeutic agents. Antimicrobial peptides can be alternative to overcome this problem because of broad- spectrum activities and their unique mode of actions, which minimizes the chance of bacterial resistance. In this book, we have discussed in detail about the discovery, classification, structural and functional diversities, modes of action and application of antimicrobial peptides. Moreover, we have also discussed on cathelicidin-derived, tryptophan-rich and proline-rich antimicrobial peptides about their sources of isolation, structure, function and their applications.