The 3 studies reported here were designed to clarify the nature of the personality trait of impulsivity. Two types of impulsivity were distinguished. Dysfunctional impulsivity is the tendency to act with less forethought than most people of equal ability when this tendency is a source of difficulty; most previous work on impulsivity appears to have focused on this trait. Functional impulsivity, in contrast, is the tendency to act with relatively little forethought when such a style is optimal. The present work indicates that these two tendencies are not highly correlated and that they bear different relations both to other personality traits and to the manner in which certain basic cognitive processes are executed.