Professional development is central to teacher competence and, with particular attention to larger-scale, structured and formal professional development interventions (PDIs), this article discusses how the impact of professional development can be conceptualized and evaluated. Various kinds of impacts on teachers, organizations and systems, and students that PDIs can have are considered, and examples from actual PDIs in the field of English language teaching (ELT) are used to illustrate how such impacts can be assessed. Throughout, various challenges that arise in trying to demonstrate that PDIs make a difference are noted, and the article concludes by suggesting some criteria that can be used to guide the development of impact evaluation frameworks on PDIs.