Publisher Summary This chapter discusses the chemical modification by active-site-directed reagents. Affinity labeling is a way of producing chemical modification of proteins in regions with specific binding and catalytic properties. In many cases, ordinary group reagents for modification are either not selective or fail to combine at active center residues, which can however be covalently modified by substrate-like reagents that localize at the active center. Because the subsequent covalent attachment is accelerated by the proximity effect so produced, the geometry of the reagent is more important than its inherent chemical reactivity. In fact, the resultant chemical reaction may be difficult to reproduce in solution. These properties emphasize the enzymic nature of this type of protein chemistry. The chapter discusses the study of isolated pure enzymes by designed reagents that generally result in inactivation. Success in this effort is adding to the number and variety of enzyme inhibitors available for quite different purposes, such as the interpretation of complex metabolic situations by selective inactivation of an enzymic component.