Understanding how enzymes catalyze specific reactions has yielded important insights into biology, provided the necessary context for the design of novel catalysts, and has facilitated the development of enzyme inhibitors for therapeutic purposes. The contributions of Professor W. Wallace Cleland helped to forge the modern framework for steady state enzyme kinetics, and together with his colleagues he made pioneering contributions to establishing heavy atom isotope effect analyses as a powerful approach for investigating the chemical mechanisms of enzyme-catalyzed reactions. Professor Cleland collaborated with and trained numerous talented enzymologists and generations of scientists. Several facets of W.W. Cleland's contributions to science are illustrated and reflected in this special issue that focuses on using kinetic isotope effects to understand chemical and enzymatic reaction mechanisms.