"Big Science" is a broad epithet that can be associated with research projects that involve huge budgets, big facilities, complex instrumentation, years of planning, and large multidisciplinary teams of researchers. Legitimizing the ESS examines the complexity of the cultural, social, and political processes from which and in which Big Science develops by focusing on the planning and development of the European Spallation Source, ESS, that is to be located in Lund in southern Sweden. Together, the chapters represent a variety of perspectives to highlight the complexity of the processes that are integral to Big Science. Thus, this volume examines the very different roles Big Science may be given in different contexts: locally, regionally, nationally, internationally, as well as historically.