This book introduces the 3R concept applied to wastewater treatment and resource recovery under a double perspective. Firstly, it deals with Innovative technologies leading to : Reducing energy requirements, space and impacts ; Reusing water and sludge of sufficient quality ; and Recovering resources such as energy, nutrients, metals and chemicals, including biopolymers. Besides targeting effective C,N&P removal, other issues such as organic micropollutants, gases and odours emissions are considered. Most of the technologies analysed have been tested at pilot- or at full-scale. Tools and methods for their Economic, Environmental, Legal and Social impact assessment are described. The 3R concept is also applied to Innovative Processes design, considering different levels of innovation : Retrofitting, where novel units are included in more conventional processes ; Re-Thinking, which implies a substantial flowsheet modification ; and Re-Imagining, with completely new conceptions. Tools are presented for Modelling, Optimising and Selecting the most suitable plant layout for each particular scenario from a holistic technical, economic and environmental point of view.