This thesis brings the concept of humanitarianism sharply into focus within the discourse of international political theory. Existing literature examines humanitarianism obliquely, via debates on military humanitarian intervention or human rights, resulting in an impoverished account of a vital idea. Meanwhile, a vibrant discussion among professional humanitarians has recently questioned the nature of their endeavour, along lines that clearly fit the remit of international political theory. Bringing together these two discussions in the course of its critical analysis, the thesis argues that humanitarianism should be conceptualised as a political context in which we articulate, negotiate and defend our understandings of common humanity. Central to this politics are the ways in which we react to and conceptualise human suffering, through humanitarian crises that are often "crises of humanity". In sparking concern and mobilising responses to suffering, the affective underpinnings of the humanitarian impulse create a complex and shifting backdrop to extensions of solidarity and humanitarian action. At the heart of this action is the idea of rescue, a crucial "presumptive occasion" of our moral life. But an important part of humanitarian action consists in the efforts to institutionalise the humanitarian impulse. In this sense human rights and projects of global justice represent important crystallisations of humanitarian concern, yet neither can fully capture the more contingent workings of the humanitarian impulse. What emerges is an understanding of humanitarianism as a broad discussion, central to the identity of contemporary liberal international political theory, but with a scope best gleaned not from cosmopolitan accounts, but from a more fluid internationalist tradition of thought. The thesis concludes that the importance of this theoretical approach will be borne out by the complex and far-reaching practical challenges that humanitarianism is set to confront over coming decades, not least the "crisis of humanity" threatened by climate change.