Hydrological drought is a textbook for university students, practising hydrologists and researchers. The main scope of this book is to provide the reader with a comprehensive review of processes and estimation methods for streamflow and groundwater drought. It includes a qualitative conceptual understanding of drought features and processes, a detailed presentation of estimation methods and tools, practical examples and key aspects of operational practice. The methods are demonstrated using sample data sets and tools that are provided on the accompanying CD. The drought phenomenon and its diversity across the world are illustrated using a global set of daily streamflow series, whereas regional and local aspects of drought are studied using a combination of hydrological time series and catchment information. The book concludes with human impacts, ecological issues and examples of procedures for designing and operating water resources schemes. The majority of the examples are taken from regions where the rivers run most of the year. The material presented ranges from well established knowledge and analysing methods to recent developments in drought research. Its nature varies accordingly, from a more traditional textbook with its clear overview to that of a research paper, which introduces new approaches and methodologies for drought analysis.