Unlike exploitative exchanges, exchanges featuring externalities have never seemed to pose particular problems to liberal theories of justice. State interference with exchanges featuring externalities seems permissible, like it is for coercive or deceptive exchanges. This is because exchanges featuring negative externalities seem to be clear cases of the two exchanging parties harming a third one via the exchange – and thus of conduct violating the harm principle. This chapter puts this idea into question. It argues that exchanges featuring negative externalities are not unjust in this straightforward way, i.e. because they would constitute an instance of wrongfully causing or risking a bodily or material harm. In fact, unless we are subscribing to particularly demanding variants of liberalism (e.g. a perfectionist liberalism as proposed by Raz) or unless we are exclusively focusing on borderline cases of externalities (i.e. of effects of exchanges hardly to be called externalities), there is no liberal theory of how exchanges featuring externalities are unjust.