Exchange involves both benefit and cost – traders take rights from others, imposing costs on them but creating benefit for themselves, but they must compensate the others for their costs by paying for the rights they've taken. External costs are imposed when one person takes another's rights without paying for them, which leads to the taker taking more rights than efficient allocation would allow and leaves the victim uncompensated for the costs of losing the rights. The law's response to this inefficiency and injustice is liability, forcing those who take rights without payment to compensate those whose rights they have taken for the costs they have borne. Thinking about how the law determines these liability prices, and how people might respond to them, reveals the underlying economic logic of liability.