For people who study and teach about international security, Barry Buzan and Lene Hansen's book is important not least because we are its central focus. It offers a sophisticated interpretation of who constitutes the “we” in security studies, how we got where we are today, what we have argued about, what made us change our minds—which did not appear to happen often—and where we might be heading. More precisely, Buzan and Hansen provide an analysis of the academic field of international security studies (ISS) as it has evolved primarily in North America, Europe, and Australasia with the goal of telling “a thorough intellectual history of how various approaches define positions with the debates about ISS” (p. 3). Although specialists from particular schools and approaches will dispute some of this book's summations and conclusions, there is no doubt that the authors have achieved their objective with admirable clarity and nuance. They do so by weaving their narrative of the field's evolution around three key analytical frameworks set out in Chapters 1–3: structuring questions, the notion of security and its adjacent concepts, and driving forces.

In terms of defining the field, Buzan and Hansen opt for a realistically broad approach that includes all those who self-identify with the label ISS. To sum up, they use a waterborne analogy to suggest that since the early years of the Cold War, the field has gone from being a single mainstream to a river delta. In more substantive terms, they define ISS with reference to four structuring questions and a range of adjacent concepts. The questions are “whether to privilege the state as the referent object, whether to include internal as well as external threats, whether to expand security beyond the military sector and the use of force, and whether to see security as inextricably tied to a dynamic of threats, dangers and urgency” (p. 10). They summarize the dominant traditional answers as defining “the state as the referent object, the use of force as the central concern, external threats as the primary ones, [and] the politics of security as engagement with radical dangers and the adoption of emergency measures.” They also note that security tended to be studied “through positivist, rationalist epistemologies” (p. 21).

The notion of adjacent concepts is necessary for this intellectual history because not all the relevant debates about the structuring questions of ISS took place through the conceptual prism of security. Buzan and Hansen therefore discuss complementary concepts, such as “strategy,” “deterrence,” and “containment,” which point to a more specific and narrow set of questions; parallel concepts, such as “power,” “sovereignty,” and “identity,” which take security into broader realms of political theory; and oppositional concepts, which work through security but arguably should be replaced by other terms such as “peace” or “risk.” With this understanding of the ISS field, the authors spend the majority of the book discussing the central arguments made by 11 distinct approaches: conventional constructivism, critical constructivism, the Copenhagen School, critical security studies, feminist security studies, human security, peace research, postcolonial security studies, poststructuralist security studies, strategic studies, and (neo)realism.

When it comes to explaining the evolution of ISS, Buzan and Hansen suggest that “five forces are particularly central to this process: great power politics, technology, key events, the internal dynamics of academic debates, and institutionalization” (p. 39). These five forces roughly map onto material power, knowledge, history, prevailing social constructions, and wealth and organizational dynamics, respectively (p. 41). They derived these five forces from their interpretation of the sociology of science literature and an inductive reading of the ISS debates themselves (p. 47).

With their analytical framework established, in Chapters 4–8, Buzan and Hansen examine the interplay between the 11 approaches to ISS in broadly chronological fashion, from the beginnings of the Cold War to the post-9/11 era. In Chapter 9, they summarize their conclusions, the principal ones being that in its contemporary river delta structure, “no ISS perspective is going to conquer the field,” and in normative terms, “it is good to have dialogue across perspectives/camps” (p. 257). With regard to the driving forces, they conclude that great power politics and technology were particularly strong during the Cold War but weaker during the 1990s. Although academic debates became more prominent in the 1980s and 1990s, Buzan and Hansen suggest that events assumed particular prominence after 2001 (p. 262). As for the future, they suggest that the conversation across the tributaries of the ISS river delta is likely to continue and should be encouraged because the strength of a field is, in large part, rooted in its ability to bring a plurality of approaches to bear on issues of common concern.

I am a bit unsatisfied that the authors' choice of the four structuring questions left out the two fundamental issues of the meaning of security itself and the agents that can best provide it. In addition, whether one thinks of the causes of war, the sources of communism/capitalism, terrorism, or climate change, it has long been clear that few threats are caused by either discrete internal or external processes alone. It therefore seems strange to make this either/or binary one of the field's foundational questions. Overall, however, The Evolution of International Security Studies is a splendid achievement. It should be essential reading for all of us who teach in the field and, hopefully, many of our students as well. It will undoubtedly provoke arguments, but that is surely the point of all good books. Moreover, for all the important differences that Buzan and Hansen ably map, their sense that the field retains many shared concerns is particularly welcome at a time when the narcissism of small differences can obscure much of what our common enterprise should be about.