DEFINING IMPULSIVITY Impulsivity can be deﬁned as “a predisposition toward rapid, unplanned reactions to internal or external stimuli without regard to the negative consequences of these reactions to themselves or others” (1). This deﬁnition encompasses the key elements of previous deﬁnitions of impulsivity (2-6). One of the reasons for multiple deﬁnitions of impulsivity has been that impulsive behavior is the expression of complex interactions between biological and environmental factors. Because of this complexity, the measurement of impulsivity has varied widely (7), and the association among various measures of impulsivity is modest. While there are a variety of techniques to measure impulsivity, one summary of this literature suggests that these various measurement techniques fall into one of three broad categories self-report, psychophysiological/biological, and laboratory behavioral measures (1). Each of these approaches to the measurement of impulsivity has its place in clinical and research domains, but each should be selected for use dependent on the setting and information sought (see Summary). In this chapter we focus our discussion on laboratory behavioral measures of impulsivity.