The study of enzyme kinetics is important from a fundamental scientific perspective, since it allows the formulation of molecular models for enzyme action, and also for technological reasons, since it allows the formulation of kinetic models for the design and evaluation of reactor performance (Messing 1975; Chaplin and Bucke 1990). The second aspect is emphasized in this chapter, which does not pretend to make an extensive review of enzymology that can be found in excellent textbooks (Laidler and Bunting 1973; Segel 1975; Dixon and Webb 1979; Marangoni 2003). As shown in Fig. 3.1, a model for enzyme reactor design or performance evaluation requires: 1) a kinetic expression of the catalyzed reaction; 2) a material balance over the process; 3) an expression for enzyme inactivation during reactor operation; 4) an expression that accounts for eventual mass transfer constraints. This chapter will analyze the first and third components, while the fourth will be analyzed in Chapter 4. All components will be put together in Chapter 5, being the objective of this chapter to establish the kinetic basis for the analysis, operation and design of enzyme reactors. Enzyme kinetics refers to the quantitative analysis of all factors that determine the catalytic potential of an enzyme. As presented in section 1.3, enzyme activity represents the maximum catalytic potential of an enzyme that is reflected by the initial rate of the catalyzed reaction. Several factors affect the expression of such potential, being the most important the concentrations of active enzyme, substrates and inhibitors, temperature and pH. In the case of insolubilized enzymes or multiphase systems, other variables that reflect mass transfer constraints must be considered.