This work presents the general biological principles--and the chemical and physical concepts-- needed for an understanding of the post- and co-translational modification of proteins, a fundamental topic in the science of protein biochemistry and molecular biology. Examples of the different types of modification reactions (glycosylation, phosphorylation, prenylation, etc.) are presented to illustrate specific points, and the book is intended to serve as a principles-based guide for studying the subject, whether in the classroom or research laboratory. In addition to its value as a timely overview of a broad, vitally important area, the book can be used in advanced courses on signal transduction, enzymology, and protein chemistry, or as a supplementary text for biochemistry or molecular biology.