This editorial situates the special issue in the context of some of the dominant understandings of teacher leadership that appear in literature that spans several decades and includes comprehensive literature reviews. Then the origins of the International Study of Teacher Leadership are outlined and the process of forming a large international research team, developing research questions, and creating a study design is explained. This is followed by an overview of the articles in the special issue that present the findings from the first two stages of the International Study of Teacher Leadership, plus a report about teacher leadership in southeastern Mexico. The editorial concludes with a caution about the potential inappropriate use of the concept of teacher leadership across different contexts.