The 5th edition of this best-selling textbook provides a systematic and comprehensive introduction to the main theoretical approaches in the study of international relations. While maintaining focus on the core theories and assessing the importance of theory in the study of International Relations, this edition has been updated throughout to take account of major events and developments, such as the Arab Spring and to reflect the developments in the field, including new material on neo-realism and neo-liberalism, post-colonialism and cosmopolitanism. Each chapter is written by a leading expert on the theory, elucidating the concepts and its application to field coverage whilst maintaining an objective perspective in their evaluations. This text can be used as reference work for particular theories, or as a tool to learn the use and importance of theory, as well as the particulars of each school of thought. This text is accessible to students on courses across the world, and it assumes no prior knowledge of any of the theories, making it the ideal companion as students begin studying theories of International Relations, whether at undergraduate or Master's level.