Suicide is the leading cause of injury mortality in the United States and the second-leading cause of death in people aged 10-34 years. While many long-term risk factors are known, the short-term prediction of suicidal behavior remains elusive. Many characteristics of suicidal behavior cut across diagnoses, but suicide is increased in recurrent psychiatric disorders, addictive disorders, and trauma-related disorders. Suicide results from the interaction of short-term and long-term behavioral regulation. The shorter the time-course of the mechanism, the closer it is to actual suicidal behavior, and the harder it is to prevent. We will discuss the manner in which impulsivity, a major determinant of short-term suicide risk, interacts with longer-term risk factors, especially sensitization to addictive or traumatic stimuli. Impulsivity predisposes to sensitization; in turn, impulsivity is a prominent component of sensitized behavior. Impulsivity can be described as a general pattern of behavior ("trait" impulsivity), as responses that are not conformed to their context (action-impulsivity), or as inability to delay reward or to take future consequences into account (choice-impulsivity). Each of these contributes to suicidal behavior. The neural mechanisms of impulsivity and sensitization are analogous, and sensitization can produce rapidly fluctuating patterns of impulsive behavior, arousal, and anhedonia. In order to recognize and prevent suicidal behavior, it is necessary to identify factors associated with susceptibility to bouts of impulsive behavior in people at elevated long-term risk.