Despite the growing (non USA) literature on the topic, neither the causes nor the effects of this democratization pattern are yet fully understood. This paper will contribute to this debate assessing the effects of the introduction of party primaries in Spain. Following the frameworks developed by Kenig (2008, 2009) and Rahat and Hazan (2010), the paper will try to establish the organisational and electoral impact of primaries in the Spanish parties. In the organizational dimension the paper will examine the outcome produced in: the competitiveness of the election (Incumbent success rate, the likelihood of contests, the number of candidates); the participation (number of participants, turnout rates, evidences of instant membership); the representation (women contesting, women being elected); and the party cohesion (factionalism). In the electoral dimension the focus will be centred in analysing: the electoral performance (share of votes and seats, changes in the institutional position); the image of the party (simpaty, etc); and the broader impact on the party system (contagion effects). The paper will analyse the party primaries held in Spain to select the party leader (ERC, ICV), the party top candidate for the national elections (PSOE, IU, ERC, UPyD), and the party top candidate for the regional elections (PP, PSOE, IU, UPyD).