This volume offers an original and theoretically grounded conceptualization and measurement of international parliamentary institutions and their role in ensuring the accountability of regional international organizations. Through a comparative analysis of the establishment, evolution, institutional organization, oversight and policymaking functions of 22 parliamentary institutions, mainly from European, African and Latin American regional international organizations, the book serves a twofold purpose. First, it allows assessment of the extent to which parliamentary institutions have (measurable) influence on the outcome of regional organizations’ decision-making processes. Second, drawing on the literature on new institutionalism and comparative regionalism, the volume investigates the conditions under which the influence of parliamentary institutions is expected to grow, thus advancing the understanding of the variation and development of this poorly explored type of international institution.