In the research literature, aggressive behavior has traditionally been classified into two distinct subtypes, impulsive or premeditated. Impulsive aggression is defined as a hair-trigger aggressive response to provocation with loss of behavioral control. Premeditated aggression is defined as a planned or conscious aggressive act, not spontaneous or related to an agitated state. The present study outlines the development of a clinically useful self-report instrument, the Impulsive/Premeditated Aggression Scales (IPAS), designed to characterize aggressive behavior as predominately impulsive or predominately premeditated in nature. The IPAS showed strong reliability and validity. Analysis of the IPASscores demonstrated thepresence of two types of aggressive behavior, impulsive and premeditated, in men referred for anger problems. The aggression of most individuals in the present sample was characterized as predominately impulsive in nature (90%).