Framed by a new and substantial introductory chapter, the book collects Stefano Guzzini's research on power, realism and constructivism. It explores the diversity of different schools and their intrinsic tensions and fallacies by analysing both theories and their assumptions, and theorists following their intellectual paths. Guzzini's approach to the analysis of power within and outside International Relations provides the common theme of the book through which the theoretical state of the art in International Relations is re-assessed. This book will be of interest to students and scholars of international relations, international politics, international relations theory and constructivism.