Both the United States (US) and Spain have offered tax expenditures to support use of wind and solar power to generate electricity at the utility scale. These tax expenditures provide an opportunity to consider design issues, the relationship between tax expenditures and other non-fiscal policy instruments and the influence of legal frameworks. The article explores tax expenditures that have been in effect in the US and Spain. In the US, the federal government since 1992 has provided an income tax credit for the production of electricity from renewable sources, and more recently the alternative of an investment tax credit. In Spain, since the 1990s there has been an important debate on the best way to promote renewable energies; different tax incentives, subsidies, and regulated prices have been used in one way or another over time. Drawing on these two case studies, the comparative analysis highlights lessons that emerge about the role of taxation as a regulatory instrument and its consistency with other policies in shifting from fossil fuels to renewable energy in order to reduce greenhouse gas emissions that contribute to climate change. These lessons can transcend national boundaries and contribute to an understanding of tax expenditures more generally.