The article explores how emission taxes can control pollution from big companies. It looks at situations where the number of firms is fixed and when firms can enter or exit the market. Surprisingly, in the second case, the best tax could be higher than the actual harm caused by pollution. This means that companies might need to pay more than the damage they do to the environment. By using a mix of a license fee and a specific tax, we can improve overall welfare compared to just using a single tax that is too low.