Policies incentivizing the private sector to reach its innovative potential in "clean" technologies are likely to play a key role in achieving climate stabilization. This article explores the relationship between innovation and cap-and-trade programs (CTPs)-the world's most prominent climate policy instrument-through empirical evidence drawn from successful CTPs for sulfur dioxide and nitrogen oxide control. The article shows that before trading began for these CTPs, analysts overestimated the value of allowances in a pattern suggestive of the frequent a priori overestimation of the compliance costs of regulation. When lower-thanexpected allowance prices were observed, in part because of the unexpected range of abatement approaches used in the lead-up to trading, emissions sources chose to bank allowances in significant numbers and reassess abatement approaches going forward. In addition, commercially oriented inventive activity declined for emissions-reducing technologies with a wide range of costs and technical characteristics, dropping from peaks before the establishment of CTPs to nadirs a few years into trading. This finding is consistent with innovators deciding during trading that their research and development investments should be reduced, based on assessments of future market conditions under the relevant CTPs. The article concludes with a discussion of the results and their implications for innovation and climate policy.climate change | emissions trading | technological change | invention | clean technology F acilitating innovation in "clean" technologies may be the key to achieving climate change stabilization without dampening economic productivity. Theory and experience indicate, however, that the private sector-apt to be the most significant source of this innovation-will not reach its full innovative potential without well-considered public policies. The pollution market failure involved in greenhouse gas (GHG) emissions implies that the development of emissions-reducing technologies will have less private value than the societal optimum. The market failure associated with innovators' difficulty in fully appropriating returns to research and development (R&D) investments-because of imitation and spillover, for example-implies that private incentives to innovate in clean technologies will be suboptimal. Another drag on private sector innovation stems from the trouble that capital markets and firms regularly have in predicting the risks and rewards of R&D investments, which can have uncertain outcomes and lengthy payback periods. Finally, there are significant challenges involved in displacing existing "dirty" technologies, which benefit from cost and performance advantages that accrue through the use of a technology over time, as well as from network externalities and the human reluctance to abandon sunk costs.Addressing the pollution market failure should spur demand for clean technology and related private sector innovation (see ref. 1 for an introduction to policies exerting a "demand-pull" on ...