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Enzyme action combines specificity with catalytic efficiency. Active sites recognize substrates, while enzyme kinetics describes how reaction rate changes as substrate concentration rises. In Michaelis–Menten behavior, the initial rate approaches a maximum as enzyme molecules become saturated; Vmax and Km characterize this relationship. Turnover and catalytic-efficiency measures offer additional ways to describe enzyme performance. At the molecular level, models such as lock-and-key and induced fit help explain substrate recognition, while transition-state stabilization and several catalytic strategies contribute to lowering activation energy. Enzyme activity also depends on conditions such as pH and temperature, and some enzymes require cofactors or coenzymes to transfer electrons, groups, or protons. Throughout catalysis, enzymes emerge unchanged and can participate in repeated cycles.