A transaction-based approach to policy intervention is presented. The approach overcomes a number of weaknesses in current approaches to policy intervention. The approach involves three main conceptual elements: transactions, transaction governance mechanisms, and governance principles. The transaction is taken to be the basic unit of analysis. Profiles of transactions vary along a number of transaction dimensions. Transaction governance mechanisms - such as, for example, government service, regulation, contracts, vouchers, markets, taxes, and self-service - are each suitable for governing transactions having particular profiles. A mechanism will fail when used to govern transactions not fitting the profile. Governance principles are criteria or expectations - for example, efficiency, justice and liberty - used to judge how well a mechanism fulfills or achieves important societal goals. Specific choices of governance mechanisms (from the set that are technically feasible) therefore should be made according to how well they satisfy these governance principles. Public high school education is used as an example to illustrate the approach. A number of conclusions are offered.