Aims – Impulsivity is a multifaceted aspect of behavior that is prominent in psychiatric disorders and has serious behavioral consequences. This paper reviews studies integrating behavioral and physiological mechanisms in impulsivity and their role in severity and course of bipolar and related disorders. Methods – This is a review of work that used questionnaire, human behavioral laboratory, and neurophysiological measurements of impulsivity or related aspects of behavior. Subjects included individuals with bipolar disorder, substance-use disorders, antisocial personality disorder, and healthy controls. Results – Models of impulsivity include rapid-response impulsivity, with inability to reflect or to evaluate a stimulus adequately before responding, and reward-based impulsivity, with inability to delay response for a reward. In normal subjects, rapid-response impulsivity is increased by yohimbine, which increases norepinephrine release. Impulsivity is increased in bipolar disorder, whether measured by questionnaire, by measures of rapid-response impulsivity, or by measures of ability to delay reward. While affective state has differential effects on impulsivity, impulsivity is increased in bipolar disorder regardless of affective state or treatment. Impulsivity, especially rapid-response, is more severe with a highly recurrent course of illness or with comorbid substance-use disorder, and with history of medically severe suicide attempt. In antisocial personality disorder, rapid-response impulsivity is increased, but rewardbased impulsivity is not. In general, impulsivity is increased more in bipolar disorder than in antisocial personality disorder. In combined bipolar disorder and antisocial personality disorder, increased impulsivity is associated with substance-use disorders and suicide attempts. Conclusions – Impulsivity is associated with severe behavioral complications of bipolar disorder, antisocial personality disorder, and substance-use disorders.