This paper studies the regulation of a good that generates an externality on consumption, where direct measurement of the externality is not feasible. I develop a method to quantify the deadweight loss of any policy that indirectly taxes the good rather than the externality, based on the R2 of a weighted regression of consumption under that policy on consumption under the first-best Pigouvian policy. I use this method to characterize the optimal indirect policy and show that it can require quantity floors and quantity ceilings. Finally, I illustrate policy implications by applying my results to the taxation of vehicle miles traveled to regulate automobile externalities.