Why do some international missions succeed at building lasting peace, while others fail? A crucial finding from the peacekeeping literature is that third-party, international involvement in war-torn countries’ peacebuilding process is crucial for success. Yet international missions from the United Nations and regional organizations are only involved in a subset of the activities that domestic actors pursue, and patterns of mission involvement vary significantly over time and from one setting to the next. This dissertation is the first to explore the sources and consequences of those divergent patterns with original data compiled specifically for this purpose and a representative case of international peacebuilding: post-conflict Kosovo. The dissertation consists of two parts. The first presents an original theoretical and analytical framework—“programmatic peacebuilding”—that both explicates the conditions under which international missions become involved in the peacebuilding activities of domestic actors and identifies the impact that such converge has on peace outcomes at various levels of analysis. The second entails an assessment of each component of the framework, using a variety of original data, including a dataset of the peacebuilding activities of all post-Cold War international missions, interviews with mission personnel, Kosovar Albanian newspaper articles, and a survey conducted in Kosovo. The first key finding is that processes of international agenda-setting and practices of cooperation, coordination, and activity sequencing are associated with increased levels of mission involvement in domestic actors’ peacebuilding activities. The second insight is that such involvement is strongly associated with many—but not all—peace outcomes assessed, including the fulfillment of activities, the implementation of peace agreements, and lower levels of conflict and violence. The third finding pertains to post-conflict Kosovo: Kosovars have developed distinct attitudes towards the international missions embedded in their country, and these attitudes greatly influence the sentiment expressed toward and trust in the country’s post-conflict institutions.