The research this book is grounded on started with the ambitious intent to understand why FEWSs may fail. However, from the beginning, the objective turned out to be challenging; first, because, so far, there is not a shared opinion on what an EWS is (among both communities of researchers and practitioners); second, as a consequence, because it is equally not clear when an EWS can be considered successful or not. The direct effect of this unclear context was that the research needed primary to face some open questions instead of going straight to the point under investigation. Specifically, the identification of what the subject of the analysis is, of which its components and functions are and of which its peculiarities (weak points) are, was primary required. Then, a first attempt to evaluate EWSs performance was possible. This book is organised according to the conceptual steps required by the research. In Chapter 1 preliminary open questions about the definition and the role of FEWSs are handled (the aim being the identification of how to evaluate EWSs effectiveness/performance). Chapters 2 to 4 focus on the real aim of the research, providing concepts and practical tools to assess FEWSs performance. Each chapter ends with a common case study describing how above evaluations can be carried out in practice. The focus of this book is flood risk, specifically, in mountain regions. However, most of results can be exported to other hazards as well.