The study explored when, under what conditions, and to what extent did European integration, particularly the European Union’s requirement for democratic conditionality, contribute to democratic consolidation in Spain, Poland, and Turkey? On the basis of a four-part definition, the dissertation examined the democratizing impact of European integration process on each of the following four components of consolidation: (i) holding of fair, free and competitive elections, (ii) protection of fundamental rights, including human and minority rights, (iii) high prospects of regime survival and civilian control of the military, and (iv) legitimacy, elite consensus, and stateness. To assess the relative significance of EU’s democratizing leverage, the thesis also examined domestic and non-EU international dynamics of democratic consolidation in the three countries. By employing two qualitative methods (case study and process-tracing), the study focused on three specific time frames: 1977-1986 for Spain, 1994-2004 for Poland, and 1999-present for Turkey. In addition to official documents, newspapers, and secondary sources, face-to-face interviews made with politicians, academics, experts, bureaucrats, and journalists in the three countries were utilized. The thesis generated several conclusions. First of all, the EU’s democratizing impact is not uniform across different components of democratic consolidation. Moreover, the EU’s democratizing leverage in Spain, Poland, and Turkey involved variations over time for three major reasons: (i) the changing nature of EU’s democratic conditionality over time (ii) varying levels of the EU’s credible commitment to the candidate country’s prospect for membership, and (iii) domestic dynamics in the candidate countries. Furthermore, the European integration process favors democratic consolidation but its magnitude is shaped by the candidate country’s prospect for EU membership and domestic factors in the candidate country. Finally, the study involves a major policy implication for the European Union: unless the EU provides a clear prospect for membership, its democratizing leverage will be limited in the candidate countries.