Political trust –the trust that people place in political institutions- seems to be critical for democracy.However, there is no agreement as to what the origins of trust are. What makes citizens trust or distrust their main political institutions? How can we explain the differences found across and within countries?These are the main questions driving this dissertation.The central hypotheses of this study are (1) that we can found differences in levels of political trust both within and across countries, (2) that, at the individual level, political trust is related mainly to perceptions of fairness and competence of national governing institutions, and (3) that, at the country level, political trust is mostly explained by different levels of corruption.In order to evaluate these claims, I analyzed political trust data from the World Values Survey of 50 democracies of the industrialized world, Central and Eastern Europe and Latin America. To evaluate country differences I used these 50 democracies.To evaluate individual differences and the individual-level correlates of trust, I focused on seven Latin American countries.The results show that, at the country-level, political trust is higher among the most developed democracies and in those countries with longer democratic traditions, parliamentary governments and majoritarian electoral systems. These variables looserelevance and statistical power, however, when the country level of corruption is introduced.Corruption is, according to the results obtained, the major factor in explaining average levels of political trust. At the individual level data, in an analysis of Latin American citizens, we found that the most important factors explaining variations in political trust are those related to the perceptions of fairness and competence of their government institutions. Trust is considerably higher when institutions are considered fair and competent in their actions. Political trust is important for governments. And citizens decide when to grant trust to their political institutions as a response to a multiplicity of factors, of which the level of corruption and the perceptions of institutional fairness and competence are among the most important ones.