The interpersonal theory of suicide proposes that acquired capability facilitates transformation of suicidal desire into lethal self-destructive behavior (Joiner, 2005). A new measure, the Acquired Capability With Rehearsal for Suicide Scale (ACWRSS), was devised to capture the key facets of acquired capability-pain tolerance and fearlessness of death-while also incorporating deliberate and active means to increase preparedness for suicide. The factor structure of the ACWRSS was tested using confirmatory factor analysis (n = 611). The 7-item ACWRSS conformed to the hypothesized 3-factor structure, demonstrating excellent fit and good internal consistency (a =. 83). Measurement invariance across gender was also demonstrated on configural, metric, and scalar levels. Next, in the first longitudinal study of the association between acquired capability and suicide ideation, intention, and readiness (n = 234), the acquired capability facets prospectively predicted specific phases in the motivational-volitional pathway toward suicide readiness. Moreover, 2 of the acquired capability components mediated the relationship between baseline nonsuicidal self-injury and suicide readiness at follow-up. In an inpatient psychiatric sample (n = 108), the ACWRSS was significantly correlated with prior suicide attempts and thoughts and episodes of nonsuicidal self-injury, and its facets demonstrated differential sensitivity to change. The ACWRSS is the first measure of acquired capability that reliably and validly captures all key facets of this critical component of the interpersonal theory of suicide. Its brevity enhances its utility for both research and clinical settings.