This chapter presents the concepts, definitions, questions and hypotheses on comparative paradiplomacy. It introduces the terminology coined and used in the literature on paradiplomacy and briefly presents the most important concepts and ideas that have been developed in the field of international relations to analyze this phenomenon, providing information on their scope and limitations. It organizes the literature in two big paradigms: one that considers that paradiplomacy can be complementary to the state’s foreign policy, and the other that proposes that these international activities can jeopardize a unified and coherent national foreign policy. It then presents five central research questions, providing tentative and testable answers, or hypotheses, to each of them. The hypotheses are organized, presented and tested organizing them according to the levels of analysis, starting with more general and parsimonious explanations (systemic). Then, to explain variations in paradiplomacy between and within countries, national variables are integrated (domestic). Finally, to explain specific outlier cases, explanations on the behavior of leaders (individual) are used. Special emphasis is placed on analyzing the interactions between systemic, domestic and individual variables.