International organizations are composed of international treaties that are linked in different ways. Institutional creation, change, death, and influence are difficult to observe without understanding this fact. We call these linked treaties "treaty complexes." One way to think of the relationship between and among treaties is via the contrasting concepts of treaty complementarity and competition. One treaty complements another treaty when it amplifies,specifies, implements, adjusts or prepares for the terms of that other document. Complementarity includes (a) building new understandings, agreements, and arrangements from an already-existent treaty, (b) more narrowly extending the contents of an existent treaty, (c) simply implementing the understandings, agreements, and arrangements already set out in an existent treaty (d)adjusting treaty design to address unintended consequences of an existent treaty and (e) preparing member states for entry into another treaty. In contrast, treaties may compete and even clash as they operate in the same issue areas, setting out different understandings, delineating divergent arrangements, or offering alternative policy responses to problems. Competition may involve treaties coexisting or entail one treaty competing with and replacing another. One can take complementarity as a cooperative agreement between treaties, while competition implies a contest. Both complementarity and competition can involve treaty replacement, the former suggesting the congruence of the treaties' contents, the latter suggesting profound divergence in the treaties' contents. International law and international relations scholars have typically been more interested in competitive relationships, notably when two different international institutions have jurisdiction or interest in a given issue area or in addressing a particular problem. Our inquiry, however, focuses on the range of complementarity in inter-treaty relationships, with our substantive interest in military alliance obligations and security treaties within the CIS and among former Soviet Union (FSU) states. We also introduce the methodology of nesting in this paper.