One of the most important strategies for protecting the environment is regulation, yet our present regulatory system is often not up to the task. An excessive reliance on "single instrument'' approaches is misguided, because all instruments have strengths and weaknesses, and because none are sufficiently flexible and resilient to be able to successfully address all environmental problems in all contexts. Accordingly, a better strategy will seek to harness the strengths of individual mechanisms while compensating for their weaknesses by the use of additional instruments. That is, in the large majority of circumstances, a mix of regulatory instruments is required, tailored to specific policy goals. We cannot assume, however, that all combinations of instruments will be better than a single instrument approach. On the contrary, different combinations of instruments, or the introduction of a new instrument to an existing policy mix, could have a variety of effects, not all of which are positive. This article examines the interactions of different categories of regulatory instruments to determine which combinations are productive, counterproductive, or context specific. The aim is to develop a prescriptive categorization of instrument mixes that will aid policymakers in policy design.