This chapter addresses the question of whether and to what degree nationalism was the cause or the consequence of imperial collapse. We go about this task in four steps. The first three steps use quantitative data that cover most of the world since 1816. In the first step, we simply analyze the temporal relationship between the transition from empire to nation-state and the foundation of nationalist organizations. We find that in the overwhelming majority, the latter precedes the former. We then proceed to a more fine-grained analysis and ask whether these nationalist organizations were perhaps inspired by imperial retreat from other parts of the empire, such that a partial imperial breakdown would further fuel the flames of nationalism elsewhere. No such effect emerges, however. In the third step, we determine whether nationalist mobilization is a cause for the individual transitions from empire to nation-state observed in the history of today's countries and find this to be the case, even if we take a host of other factors into account (including the weakening of empire through wars).In the fourth and most important step, we go beyond these coarse quantitative analyses and shift to empires as units of analysis, asking to what degree nationalism caused imperial collapse. Discussing the d e m i s eo ft h eO t t o m a n ,H a b s b u r g ,F r e n c h ,B r i t i s h ,P o r t u g u e s ea n d Soviet empires, we assess the varying degrees to which nationalist movements contributed to each imperial collapse and the extent to which each transition was due to other, unrelated factors, including the voluntary retreat of empire or the breakdown of empire due to defeat in international wars. We find that in all cases of imperial collapse nationalist movements played an important, and sometimes, crucial role (for the opposing view, see Betts