If one step in a reaction mechanism is much slower than all other steps, it is the rate-determining step. The rate-determining step is the slowest step in a reaction mechanism and determines the overall rate of the reaction. This is because the rate of the reaction cannot be faster than the rate of the slowest step. The other steps in the mechanism may occur more quickly, but their rates are limited by the rate of the rate-determining step. Therefore, understanding the rate-determining step is important for predicting the rate and outcome of a chemical reaction.