This article compares the processes of democratization in Spain and Mexico. The central arguments are: first, in order to understand the process of democratization, it is essential to look at the process of contestation-mobilization from below that preceded democracy. Second, the level of institutionalization of the authoritarian regimes is a key variable in explaining the relatively 'rapid' democratization of the Spanish regime and the protracted transition taking place in Mexico. Third, that this level of institutionalization also helps us to understand the strength or weakness of the authoritarian elites. Fourth, the contestation-mobilization process is linked to the process of organization of the political parties of the opposition in the democratization process. Finally, although in Spain the opposition political parties, in particular the Partido Socialista Obrero Espanol was able to become a governing alternative, in Mexico the structure and strength of the regime have slowed the possibilities of alternation and more generally, of a full transformation of the authoritarian regime into a competitive and inclusive democracy.