Most energy companies know that governments will—sooner or later—impose limits on their emissions of greenhouse gases. A small but growing number— including two oil majors and some big U.S. utilities—are already preparing for the inevitable. They are taking voluntary steps to reduce the amounts of CO2 and pollutants their activities generate, and experimenting with market-based and internal programs for trading emissions credits for multiple greenhouse gases. Such proactive approaches do more than lend needed certainty to corporate environmental planning; they also promise to give their ‘green’ adopters a competitive edge, in the form of early practical experience with emissions measurement and trading and compliance risk management.