The demand for air quality depends on health impacts and defensive investments, but little research assesses the empirical importance of defenses. A rich quasi-experiment suggests that the Nitrogen OxidesWillingness to pay (WTP) for well-being frequently depends on factors that enter the utility function directly (e.g., the probability of mortality, school quality, local crime rates, etc.) and compensatory investments that help to determine these factors (Grossman 1972). In a wide variety of contexts, the empirical literature has almost exclusively focused on the direct effects (e.g., health outcomes) of these factors and left the defensive investments largely unmeasured. As examples, there has been little effort to measure: the use of medications or air filters to protect against poor air quality (e.g., Chay and Greenstone 2003; Currie and Neidell 2005); parental expenditures on supplemental tutoring to improve educational outcomes for their children; or the costs of alarm systems and additional security to protect against crime. All of these defensive investments are costly and displace consumption of utility-generating goods. Indeed, economic theory suggests that these actions * Deschênes: Department of Economics, University of California, 2127 North Hall, Santa Barbara, CA 93106, IZA, and NBER (email: olivier@econ.ucsb.edu); Greenstone: Department of Economics, University of Chicago, 1126 E. 59th Street, Chicago, IL 60637, and NBER (email: mgreenst@uchicago.edu); Shapiro: Department of Economics, Yale University, 37 Hillhouse Avenue, New Haven, CT 06511, and NBER (email: joseph.shapiro@ yale.edu). This paper was accepted to the AER under the guidance of Hilary Hoynes, Coeditor. We are grateful to numerous seminar participants and colleagues, especially Christopher Knittel, for insightful comments. Nick Muller generously helped with CRDM, Dan Feenberg, Mohan Ramanujan, and Jean Roth gave considerable help with MarketScan data,