This book is a synthetic historiography of present-day international relations theory, a critical analysis of the continuing diversity and complexity of enduring themes through a sustained focus on the analysis of the empirical evidence accumulated by social scientists. Special attention is given to key historical changes in theoretical approaches over the past half-century with full recognition of the contestation over state-based theory, and the changing fortunes of contemporary approaches. The book suggests that viable theories must transcend current intellectual fashion, and attempts to bring together theory and practice while demonstrating the difficulty of assessing competing theories. It addresses multiple strands of thought and assumes that their development cannot be understood in isolation from each other.