This article describes the development and validation of a measure for transformational and transactional leadership that may be used as part of a more comprehensive instrument to measure a school’s capacity to implement school-based interventions. Data used for this study were gathered from the reports of 1144 teachers from 45 Norwegian schools who were taking on a school development intervention aimed at preventing and reducing problem behaviours among students. Using exploratory factor analysis and confirmatory factor analysis, measurement structures with two factors at the teacher level and one factor at the school level provided a good fit to the data. Although there is some overlap between transactional and transformational leadership, the two concepts do measure distinct features. Transformational and transactional leadership was eventually measured by teacher reports on two scales, each consisting of four items. As part of the validation, scores on the scales were related to other aspects relevant for organisational capacity. As expected, the transformational and transactional leadership scales both correlated significantly negatively with staff freedom and positively with innovation climate, affiliation, principal positions in schools, and collaborative activity at both the teacher and the school levels.