This paper studies the effect of emission limits and emission taxes on a firm that emits two pollutants. If both pollutants are regulated separately, emission standards are strategic complements while emission taxes are strategic substitutes. Emission standards and emission taxes are equivalent, if the government imposes tailor-made emission standards or taxes on each pollutant separately. If the government is restricted to impose a uniform emission standard or a uniform tax, however, welfare is higher under the emission standard.