This paper presents a fresh conceptualisation of teacher leadership in which teachers take centre stage in educational reform. It draws on evidence from the International Teacher Leadership (ITL) project to support a theory of educational reform in which school improvement is achieved by mobilising the massive untapped potential of teachers as leaders of innovation. Here teacher leadership is conceived as a key dimension of teachers’ professionality rather than a function of special roles and responsibilities within organisational structures. Evidence to support this argument is drawn from the ITL project in which partners in 15 countries (Albania, Bosnia & Herzegovina, Bulgaria, Greece, Croatia, Kosovo, Macedonia, Moldova, Montenegro, New Zealand, Portugal, Romania, Serbia, Turkey & UK) established programmes of support for teacher leadership and participated in an action research-based evaluation process. The article explores the data in order to illuminate the nature of the intervention and to demonstrate how teachers were able to contribute to change and improvement in their schools. The paper also draws on evidence from the ‘Future of the Teaching Profession’ survey commissioned by Education International to amplify the argument about the potential for the development of teachers’ professionality.