In this study, we report on the development and psychometric evaluation of the Risk-Taking (RT) and Self-Harm (SH) Inventory for Adolescents (RTSHIA), a self-report measure designed to assess adolescent RT and SH in community and clinical settings. 651 young people from secondary schools in England ranging in age from 11.6 years to 18.7 years and 71 young people referred to mental health services for SH behavior in London between the ages of 11.9 years and 17.5 years completed the RTSHIA along with standardized measures of adolescent psychopathology. Two factors emerged from the principal axis factoring, and RT and SH were further validated by a confirmatory factor analysis as related, but different, constructs, rather than elements of a single continuum. Inter-item and test-retest reliabilities were high for both components (Cronbach's [alpha = 0.85, r[subscript tt] = 0.90; Cronbach's [alpha] 0.93, r[subscript tt] = 0.87), and considerable evidence emerged in support of the measure's convergent, concurrent, and divergent validity. The findings are discussed with regard to potential usefulness of the RTSHIA for research and clinical purposes with adolescents. (Contains 1 figure and 7 tables.)