This is a major new edition of a highly-regarded textbook on International Relations theory which combines deep analysis into the diversity of thought within the major scholarly traditions and the guidance for students on doing their own theorising. Knud Erik Jorgensen analyses the nuances of the main contending theories and approaches, their philosophical underpinnings, and explains their use and relevance to different research agendas. This is all placed within the context of cross-cutting coverage of key current issues and debates; of the philosophical foundations of IR theory; and of why different theories are addressed to different research agendas. All chapters have been fully revised and updated, and a new chapter on the Human-Nature tradition has been included to reflect the changes within the field. This text is the most up-to-date and informative text on International Relations theory, and is an essential companion for all International Relations students.