This chapter sets out the theoretical paradigm through which this comparative study of intergovernmental agreements will proceed. It defines the object of this analysis, intergovernmental agreements. A decision between officials to keep each other informed of new agricultural regulations could count as either an informal agreement of intergovernmental relationships. The feature of definition of intergovernmental agreements is the requirement that they must be concerned with a 'single policy project'. An institutional perspective will be proposed as the most appropriate means of defining agreements in a theoretical sense. Institutionalism will serve as the theoretical basis for the hypotheses that seek to explain why certain federations form more agreements than others. The chapter argues that institutionalism is also the best way to understand the formation of agreements, compared to competing alternatives of the sociological and rational-choice approaches. It concludes by outlining the logic behind the selection of the six federal systems that will be used to examine the institutional theory provided here.