This article introduces the special issue on Trump and unpredictability in international relations. It analyses each of the contributions to the special issue in turn, as they appear in the running order, while also elaborating on the intellectual and political context for the special issue as a whole. Paralleling the special issue, this introduction begins by setting out the problem of understanding the very notion of unpredictability itself as a general concept; turns to unpacking that theoretical complexity in more empirical terms by questioning how we can understand unpredictability in relation to Trump's foreign policy; explores the contestation of that understanding by analysing alternative explanations for unpredictability and whether this constitutes doctrine; then delves more deeply into the specific notion of Madman theory, which comprises a critical and highly prominent sub-section of the unpredictability debate; and culminates in a more expansive focus on the impact of unpredictability as Trump's foreign policy approach, specifically in relation to China.We first approached the editors of the Cambridge Review of International Affairs with the outlines of a special issue in October 2019. By that point, Trump had served nearly three years of his four-year termand political scientists, pundits, and journalists had grown accustomed to living in a world where any moment their phones could buzz with news of a bizarre tweet that could create a potential foreign policy crisis. It was now widely recognised that the Commander-in-Chief was erratic compared to his predecessors and conventional expectations of presidential behaviour. Analysts even began talking about Trump's unpredictable activity as a method of foreign policy doctrinemore specifically, the 'doctrine of unpredictability' (Fuchs 2017). The president's supporters and defenders argued that this approach could shake up international politics for the better, forcing allies to stop taking American leadership and largess for granted (Krauthammer 2017). Detractors claimed it would lead to chaos, destabilize key relationships, and ultimately disrupt the American-led liberal international order (Nedal and Nexon 2017; Saletan 2016).