A purposeful sample of 21 school leaders from the UK and abroad were interviewed about their use of Twitter. The Twitter timelines of these respondents were also analysed. The study was framed around four research questions designed to interrogate the issues surrounding senior leaders' use of Twitter. The data collected pointed towards the growing importance of Twitter as a forum for discussion about a whole gamut of issues related to education and leadership. The research uncovered important ways in which Twitter is being used to supplement, or in some cases replace, traditional modes of professional development. This is seen to have implications for the way leaders' careers evolve over time. A revised model of leadership career progression is proposed. The revised model provides a conceptual framework for charting social media engagement amongst leaders as their careers progress. By systematising social media engagement in this way, the study makes an important contribution to the corpus of knowledge that already exists in relation to social media use in educational settings. Practical implications include, amongst other things, suggested changes to the professional development of leaders and a call to greater awareness of social media amongst leaders themselves.