This significant volume provides reviews on protein methylation reactions which occur ubiquitously in nature. Written in a comprehensive style, this work covers knowledge on the natural occurrence of methylated amino acids, their chemical synthesis, plus the enzymology, biological regulator, and biological significance of free methylated amino acids. It also includes various specific biological systems where posttranslational methylation reactions have been demonstrated. This text is unique because the chapters are critically reviewed by authorities in each respective field. This well-written volume is an excellent reference to researchers as well as graduate students interested in post-synthetic modifications of macromolecules.