This chapter develops and tests hypotheses about possible influences that lie outside national borders. There are many good reasons to expect that domestic factors are not the sole determinants. We lay out a theoretical framework that systematically catalogues most of the possible international hypotheses: exogenous shocks and endogenous networks such as those linking neighbors, allies, and colonizers and colonies. We then test selected hypotheses about exogenous shocks and contagion – the spread of democracy outcomes from country to country through various international networks. Surprisingly, contagion at first appears to be real but so small that it could be ignored when studying domestic influences. However, for some kinds of contagion our analysis implies that the long-run effects grow quite large and must be taken into account if we want to understand how democracies develop and decline. This paradox leads us to conclude that international influences are a hidden dimension of democratization.