Although in treating adolescents with problem behaviour emphasis is placed on teaching skills of immediate relevance to everyday prosocial functioning, instruments for assessing these skills have been lacking. This article describes the development and preliminary validation of an instrument that enables group care workers to assess skilful behaviour in adolescents in residential treatment facilities: the Adolescents' Tasks and Skills Questionnaire (TASQ). TASQ was constructed using data on 567 adolescents in Dutch residential treatment facilities. TASQ's 29 scales provide insight into specific skills and comprise six wide-ranging competency dimensions. Internal consistency is satisfactory and the preliminary validation study yielded promising results. © 2009 Taylor & Francis.