This chapter studies paradiplomacy comparatively within a country, Mexico, analyzing the international relations of its 32 states. It explains the dramatic increase of paradiplomacy in Mexico through a combination of systemic international variables (globalization and interdependence), and institutional domestic variables (democratization, decentralization and structural reform), using a veto points and players model. It describes the Mexican constitutional and legal framework for IRSSG and, based on it, the Mexican Paradiplomacy Index (PI-Mex) is constructed to quantify the level of international activities of Mexican states, considering the institutionalization of these actions at the local level, and the economic and political activities conducted abroad. The PI-Mex is measured using three moments in time, with five-year intervals (2004, 2009 and 2014), to assess how paradiplomacy in Mexico has changed during the last decade. Finally, three domestic variables at the sub-state level are used to explain the variation in the PI-Mex: gross state income, juxtaposed government and border location. Using a simple statistical model (OLS regression), the income variable is tested, finding that there is more paradiplomacy when the economic resources at the state level are larger. The existence of juxtaposed government and a geographical border location also generate incentives for SSGs to conduct more paradiplomacy.