This article considers how leaders' personality traits change over time. I focus on how leaders become more authoritarian, overconfident and more mistake-prone; how, when and why do leaders ‘break bad’? Temporal evolution of leaders is an important topic given the long tenure of many political leaders and the influence these leaders have over policies, including foreign policies. There is very little work on how leaders' personalities develop and how they interact with changing constraints and opportunities. This article is an agenda-setting review, designed to push foreign policy analysis in new directions. This is especially important given the resurgence in research on personalities and the renewed interest in leaders. Drawing on diverse and multi-disciplinary scholarship on the psychological effects of aging, experience, learning and power-holding, this article develops expectations about leader personality change. I discuss challenges for research in this area, focusing on how ‘bad’ can be conceptualized, and offer specific avenues for future investigations.