This chapter introduces the main objectives of this report. The report, based on a series of in-depth case studies, examines the impacts on environmental effectiveness and economic efficiency of using an “instrument mix”, rather than a single instrument, to address a given environmental problem. The case studies review instrument mixes applied in OECD member countries to address household waste, non-point sources of water pollution in agriculture, residential energy efficiency, regional air pollution and emissions to air of mercury. The report examines how member countries assess the environmental effectiveness and economic efficiency of a given instrument or instrument mix. It analyses the additional impacts on environmental effectiveness and economic efficiency caused by the combination of instruments, and asks which types of instrument mixes are likely to provide the best results.