This essay reviews recent developments in liberal international relations theory, both empirical and normative. Furthermore, we seek to highlight parallels between contemporary liberal scholarship on international relations and the thought of classical figures such as John Locke, Adam Smith, Immanuel Kant, Giuseppe Mazzini, and John Stuart Mill. In the first part of the essay we introduce key liberal principles and ideas and identify three different traditions of liberal thought on international relations. Thereafter we discuss the relationship between liberal democracy and international peace, followed by an overview of related scholarship on cooperation among democracies. In the final part of the essay, we briefly discuss two alternative liberal approaches to the ethics of military intervention: we show that although liberal theorists all share a fundamental attachment to representative governance and human rights, they can fundamentally differ in their support for coercive regime change.