Book excerpt: Even though they are present in nature, non-proteinogenic amino acids are usually defined as unnatural or non-natural. Beside their structural diversity, interest in these compounds is due to their occurrence in nature, their biological properties, the analytical aspects, their use as probes, and their incorporation into peptides and proteins, among other reasons. Divided into five convenient sections, Unnatural Amino Acids: Methods and Protocols deals with enzymatic methods used to produce non-natural amino acids, aspects concerning the presence of unnatural amino acids in peptides with antimicrobial properties, genetic incorporation of unnatural amino acids into proteins (yeast and mammalian cells), and detection and quantification of D-amino acids and related enzymes. Written in the highly successful Methods in Molecular BiologyTM series format, chapters contain introductions to their respective topics, lists of the necessary materials and reagents, step-by-step, readily reproducible laboratory protocols, and notes on troubleshooting and avoiding known pitfalls. Authoritative and accessible, Unnatural Amino Acids: Methods and Protocols serves as an ideal guide for scientists and contributes to directing the attention of researchers to the many fields of growing scientific interest in non-natural amino acids.