This dissertation contributes to an improved understanding of the causes and consequences of border change by examining the following two questions: Why have some borders remained more stable than others? And how does border change affect the risk of conflict between and within states? Building on a recent strand of the literature that treats borders as institutions, I develop a set of arguments that help explain when and where border changes occur and how these changes affect the risk of conflict. To test these arguments, I rely on a new geocoded dataset that maps state borders from 1886 to the present. The first part of the dissertation deals with the formation and change of international borders. I argue that most borders are the outcome of interstate bargaining, in which states compete over territory but aim to reach a stable settlement, as failing to do so would entail considerable uncertainty and a risk of incessant conflict. To find agreements, states commonly rely on established focal principles, such as the use of natural features, historical precedents or cartographic referents as the basis for new borders. While each of these principles facilitates agreement, not all of them result in borders that are equally well-defined. Straight borders derived from cartographic lines are less recognizable on the ground than borders that follow clear-cut natural features of historical precedents. If the location of borders remains less clear to people on the ground, it is less likely that borders effectively coordinate interstate relations, which reduces the opportunity costs of redrawing them. In an analysis of border durability since 1886, I find that straight border segments have been less durable than irregular ones, while borders that follow prominent natural features or historical precedents have been more likely to persist. In the second part, I examine the persistence of border instability in a first global analysis. Borders have lasting effects on individual behavior and identities, which makes it increasingly difficult to redraw them, the longer they have remained in place. However, this also suggests that border instability may persist for long periods of time. In historically unstable regions, existing borders are less firmly institutionalized, while previous borders may effectively compete with current ones as the commonly accepted divide, which increases the risk of further instability. To test these arguments, I rely on geocoded data on interstate territorial disputes, and introduce a new dataset that maps the territories claimed by separatist movements since 1946. I find that regions with unstable borders prior to World War II have remained more prone to interstate territorial disputes, domestic disputes and border change in the period thereafter. The third part adopts a broader regional perspective and examines the spillover effects of border change. Borders have been kept in place in part by international rules and norms aimed at reducing instability and uncertainty in international politics. However, these rules can and have been violated, and are not consistently enforced across time and space. Potential revisionist states base their choice to initiate a dispute in part on the observed behavior of other, proximate states. Successful border changes nearby demonstrate the weakness of international constraints, suggesting favorable conditions for revisionism. In a dyadic analysis of territorial changes and disputes since 1816, I find that territorial changes between one pair of states increase the risk of new disputes between other, neighboring states. This effect is largely driven by conflictual instances of territorial change, but is also mostly limited to the pre-World War II period.