Assessment of ecological status and quantification of its relationships with anthropogenic pressures critically depend on knowledge of relevant biotic and abiotic settings and processes. Needs raised by the implementation of WFD have urged scientific research in many fields in order to find more stable and robust biological metrics and to diminish the uncertainty of assessment results. The number of WFD related scientific publications has increased linearly and exceeded 1500 in the year 2008. This book comprises a small set of this large flow of publications and is mainly focused on the development of biological assessment methods and intercalibration of the assessment results.