Emissions trading is becoming increasingly popular in environmental law. Allowances to trade emissions can either be auctioned off or handed out free of charge by means of grandfathering. Although grandfathering is frequently used in emissions trading schemes, it is a popular view in the economic and legal literature that grandfathering is inconsistent with the polluter-pays principle. We come to a different, more nuanced view. The question of whether polluters pay under grandfathering depends on how the polluter-pays principle is interpreted. We present a taxonomy of interpretations. Based on an efficiency interpretation of the principle, consistency is demonstrated by emphasizing the economic impact of the opportunity costs of gratis allo...