Enzymes are biological catalysts. Recall that a catalyst is a substance that accelerates a chemical reaction without itself being consumed by the reaction. Enzyme catalysis holds a special place in the history of chemistry. The law of mass action was enunciated in the nineteenth century in a thermodynamic context by Guldberg and Waage. By the early twentieth century, the law of mass action was supported by a growing mass of both thermodynamic and kinetic evidence. Nevertheless, some reactions had complex rate laws which were not obviously compatible with mass action. Enzyme catalysis posed such a difficulty. In fact, there was some debate as to whether enzyme molecules even had to come in contact with the reactant molecules to accelerate the reaction. In 1902, Victor Henri showed, with some help from Max Bodenstein, that enzyme kinetics is in fact compatible with the law of mass action. The complex rate law obeyed by enzyme-catalyzed reactions was simply a manifestation of a complex mechanism. Although debates with respect to the mode of action of enzymes persisted for some time, it was at least clear from this point on that enzyme catalysis was compatible with the then-emerging theory of chemical kinetics. This is the vein we will pick up in this chapter. Our study of enzyme kinetics will particularly emphasize the derivation of rate laws and the analysis of data from initial rate experiments.