The main goal of this study is to investigate the transformation of the Spanish penitentiary system in the transition from dictatorship to democracy. This work partly deals with juridical sociology and partly with comparative juridical sociology. It was carried out by collection of data directly from a cluster of prisoners y a cluster of penitentiary operators in the prison of “Quatre Camins” in Barcelona – Spain and approached analysing the possible relationships between two antithetical models of political systems and two models of penal system. We have tried to give a social and political overview answering questions such as: “How does a penitentiary system changes in the transition from dictatorship to democracy?; What aspects does it preserve and what does it refuse or transform?; Are modern prisons truly inspired to democratic principles?; What is the social depiction of the prison and above all in how many different ways is it depicted in the collective imagination?” The common sense and the empirical researches indicate a significant difference between the treatment of the prisoners in the democratic regimes and in the totalitarian regimes. In the democratic regimes it is possible to monitor the prison from the outside and to denounce any pathology existing within. In the totalitarian ones the universe of detention is dull and is entrusted to the discretion and the whim of the authority. The survey carried out suggests further indications. Apart for few changes in terms of quality, some modifications in terms of quantity emerged from the data collected. These changes mark the transition from dictatorship to democracy in Spain. As for quality they concern the function of the prison and the treatment of the prisoners. As for quantity, nowadays as was the case in the period of Franco’s regime, it is difficult for the democratic system to manage the overcrowding of prisoners of greater dimensions respect to the previous ones. The empirical research shows that the overcrowding of penitentiary institutes limits the rehabilitative aim of the prison as the individual treatment and consequently some human rights of prisoners.