The "safety valve" is a possible addition to a cap-and-trade system of emissions regulation whereby the authority offers to sell permits in unlimited amount at a pre-set price. In this Way the cost of meeting the cap can be limited. It vas proposed ill the US as a way to control perceived high costs of the Kyoto Protocol, and possibly as a way to shift the focus of policy from the quantity targets of the Protocol to emissions price. In international discussions, the idea emerged as a proposal for a compliance penalty. The usefulness of the safety valve depends on the conditions Under which it might be introduced. For a time it might tame all overly stringent emissions target. It also call help control the price volatility during the introduction of gradually tightening one, although permit banking can ultimately serve the same function. It is unlikely to serve as a long-term feature of a cap-and-trade system. however, because of the complexity of coordinating price and quantity instruments and because it will interfere with the development of systems of international emissions trade.