G creates too many incentives to adopt the new technology. · The first-best supply policy is time-consistent if there is a very large number of firms. If there are relatively few firms, the first-best supply policy may not be time-consistent, and the regulator must ratchet the supply of permits. With this policy, there are not enough incentives for firms to adopt the new technology. The results do not strongly favor one policy instrument over the other, but if the point of an emissions trading program is to increase technological efficiency, it is necessary to continually adjust the supply of permits in response to technological change, even when damage is linear. This continual adjustment is not needed for an emissions tax when damage is linear, which may give emissions taxes an advantage over emissions trading. This paper - a product of Infrastructure and Environment, Development Research Group - is part of a larger effort in the group to study the economics of pollution control. Copies of the paper are available free from the World Bank, 1818 H Street NW, Washington, DC 20433. Benoît Laplante may be contacted at firstname.lastname@example.org.