In discussions of a cap-and-trade system for implementation of Kyoto Protocol-type quantity targets, a "safety valve" was proposed where, by government sales of emissions permits at a fixed price, the marginal cost of the effort could be limited to a predetermined level. The advantages seen for such a hybrid system included the shifting of the Kyoto architecture toward a price-based system, and the blunting of opposition to the Protocol on the basis of anticipated high cost. This paper reviews the theoretical underpinnings of the preference for a price instrument for controlling stock pollutants like greenhouse gases, and summarizes the arguments supporting and opposing the safety valve idea within the policy debate. If, in the face of uncertainty, emissions are to be limited to a fixed quantity target, then some means needs to be provided to avoid complete inflexibility. A safety valve can serve this function, although similar advantages can be achieved by the phasing in of quantity targets, coupled with provision for banking and borrowing.