A new line of work on deterrence began emerging after the end of the Cold War and gained momentum after the September 11 terrorist attacks. Building on a previous characterization by Robert Jervis that identified three waves of deterrence research, 1 this work is here designated the fourth wave. The fourth wave reflects a change from a focus on relatively symmetrical situations of mutual deterrence to a greater concern with what have come to be called asymmetric threats. The most important result has been to reveal the value of adopting a broader concept of deterrence that is not exclusively military in nature.The initial wave of deterrence theorizing came after World War II and was driven by the need to respond to a real-world problem -the invention of the atom bomb. The second wave emerged in the 1950s and 1960s. It applied tools like game theory to develop much of what became conventional wisdom about nuclear strategy (at least in the West). Starting in the 1960s but really taking off in the 1970s, the third wave used statistical and case-study methods to empirically test deterrence theory, mainly against cases of conventional deterrence. The case-study literature also challenged the rational actor assumption employed in second-wave theory.For at least a decade after Jervis published his seminal review, most research involved 'normal science' contributions within the second and third waves, along with lively debates across the existing theoretical and methodological divides. Even after the end of the Cold War and the rise of concerns about rogue states and terrorism, scholars have continued to produce research that adds to and in some cases challenges previous work in the first three waves.3 Nearly all of this research, however, retains a focus on traditional interstate conflict. Given that it has a different substantive focus than work that addresses asymmetric threats, recent scholarship that deals with deterrence in the context of great power relations or regional rivalries will not be considered part of the fourth wave.Alongside ongoing work that grows out of the second and third waves, a distinct new strand of deterrence research has emerged. Like the first wave, this fourth wave is primarily a response to real-world developments -most notably, 9/11. For this reason, again similar to the first wave, the latest wave has been more concerned with developing deterrence strategy than deterrence theory. Attempts to test fourthwave ideas empirically have also been limited. Instead, the work has mainly been conceptual and policy-oriented.The roots of the fourth wave can be traced to the collapse of the Soviet Union. By the second half of the 1990s, studies began appearing that focused on post-Cold War deterrence, 4 including some important pre-9/11 attempts to think about how to deter terrorism involving weapons of mass destruction (WMD). 5 The terrorist attacks of