Microorganisms are capable of producing a wide variety of biopolymers. Homopolymer peptides, which are made up of only a single type of amino acid, are far less ubiquitous. The only two amino-acid homopolymers known to occur in nature are presented in this volume. Poly-epsilon-L-lysine is a polycationic peptide and exhibits antimicrobial activity against a wide spectrum of microorganisms. It is both safe and biodegradable and is therefore used as a food preservative in several countries. In addition, there has been great interest in medical and other applications of poly-lysine and its derivatives. In contrast, poly-gamma-glutamic acid is an unusual anionic polypeptide. It is also water soluble, biodegradable, edible, non-toxic and non-immunogenic and can be chemically modified to introduce various drugs. These features are very useful for pharmaceutical and biomedical applications. Poly-glutamic acid is also a highly attractive as a food ingredient.