Environmental economics has made it possible to estimate prices for air pollution externalities. However, these values are rarely observed in emissions trading markets. Moreover, market outcomes show prices persistently remain below expectations and frequently fall over time. Low allowance prices may appear virtuous, but often reflect poor market design that does not anticipate interaction with other policies, and may undermine confidence in market-based approaches to environmental policy. This paper surveys emissions markets and factors influencing prices, and concludes with a discussion of how market design can anticipate and remedy the strong tendency for low prices.