Flexible links between sensory stimuli and behavioral responses underlie many cognitive processes. One process that contributes to flexible decision-making is categorization. Some categories are innate or overlearned, but, in many cases, category boundaries represent flexible decision criteria that can shift on the fly to adapt to changes in the environment. The ability to shift category boundaries allows decision-making to adapt to changing circumstances. We found that monkeys were able to switch rapidly between two category boundaries when classifying the speed of a moving dot pattern and that neurons in monkey frontal eye field (FEF) changed their activity when the boundary changed. The responses of a subpopulation of FEF neurons that were sensitive to both stimulus and boundary speed were used to classify the stimuli as accurately as the monkeys' performance.