Prodrugs are substances administered in an inactive form that is then metabolized in the body in vivo into the active compound. The rationale behind administering prodrugs is to optimize absorption, distribution, metabolism, and excretion of these drugs. Since first described in the 1950s, prodrugs continue to be a fertile area of research. There are a number of small pharmaceutical/biotech companies dedicated to using prodrugs for the delivery of older but problematic drugs as well as to developing broad-based prodrug technologies for application to new and future drugs. These volumes represent a comprehensive guide to prodrugs and will guide the reader through the current status of the prodrug concept and its many applications and to highlight its many successes in overcoming formulation and delivery of problematic drugs.