What are the impacts on environmental effectiveness and economic efficiency of using an "instrument mix," rather than a single instrument, to address a given environmental problem? What are the main arguments for using such instrument mixes, and are the instrument mixes currently in use actually well designed in response to these arguments? These are the main questions addressed in this report, which is based on a series of in-depth case studies. The case studies analyse instrument mixes applied in OECD 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.