The aim of this guidebook is to become a reference tool for noise mitigation through a better use of Noise Reducing Devices (NRD) (e.g.: Noise Barriers or sound Absorptive Claddings). It targets all the stakeholders involved in NRD projects (designers, manufacturers, authorities, construction companies, maintenance companies...) and who are willing tooptimise the products and their use, either for rail or road noise mitigation. This guidebook is based on the final results and outcomes of the QUIESST research: it synthesizes this 3-years project and provides examples of best practices and recommendations on: • The effect of sound reflections in the far field by the definition and the determination of intrinsic far field performance indicators; • A new method for in-situ measurements of sound reflection and airborne sound insulation of NRD; • A better knowledge of the European NRD market through the first database comparing 400 different devices and their acoustic performances through more than 1400 test reports • The holistic approach on how to optimise NRD at 3 different levels: o Intrinsic (reflection, transmission and diffraction indices); o Extrinsic / holistic (acoustic, economic and environmental performances); o Global impact on typical dwellings; • The NRD sustainability: how to design, build, maintain and decommission better sustainable NRD.