"This paper develops a simple model of a polluting industry and an innovating firm. The polluting industry is faced with regulation and costly abatement. Regulation may be taxes or marketable permits. The innovating firm invests in R&D and develops technologies which reduce the cost of pollution...

"This essay revisits the question of instrument choice for the regulation of externalities in the context of climate change. The central point is that the Pigouvian prescription to equate marginal control costs with the expected marginal benefits of damage reduction should guide the design of...