Integrated assessment of running waters is the focal point of the European Water Framework Directive. This book provides deep insight into principles and methodologies of river assessment, covering the whole range from the definition of reference conditions and river typologies to specific problems such as the most appropriate taxonomic resolution and software applications. It is focused on benthic macroinvertebrates, the taxonomic group most frequently used in bioassessment. The underlying principles are relevant for other taxonomic groups used in bioassessment and other ecosystem types, even beyond aquatic habitats.