The aim of this chapter is to explore the contribution of pacifism to international relations (IR), and in particular to demonstrate its relevance to discussions around peacebuilding, statebuilding, and peace formation. The chapter argues that despite its currently subjugated status within IR, as a form of theory rooted in a real-world critique of violence and its effects and in the historical practices of peacemaking and nonviolent resistance, pacifism is ideally placed to offer insights and suggestions for both practitioners and theorists of peacebuilding. The chapter begins by explaining what pacifism is, its main types and approaches, and offers a brief outline of the history and legacy of pacifism in IR, some of the main objections to pacifism, and some answers given by pacifists to these criticisms. The chapter goes on to explore what pacifism can contribute to IR by way of, first, a critique of violence, power, and just war theory and, second, positive contributions to discussions of power and agency, security, civilian protection, peacebuilding, statebuilding, and peace formation. The chapter makes the case that pacifism is a credible and insightful approach to IR that should be taken far more seriously than it is. Moreover, the current historical juncture provides an ideal moment for the return of pacifism to IR, although there are a number of serious challenges it will first have to surmount.