The authors describe the development of the Suicide Attempt Self-Injury Interview (SASII), an instrument designed to assess the factors involved in nonfatal suicide attempts and intentional self-injury. Using 4 cohorts of participants, authors generated SASII items and evaluated them with factor and content analyses and internal consistency statistics. The final measure was assessed for reliability and validity with collateral measures. The SASII assesses variables related to method, lethality and impulsivity of the act, likelihood of rescue, suicide intent or ambivalence and other motivations, consequences, and habitual self-injury. The SASII was found to have very good interrater reliability and adequate validity.