In a large number of products liability lawsuits, sellers assert that plaintiffs' claims should be rejected because their products fall under some federal regulatory regime, and that the regulatory statute takes precedence over or "preempts" state tort law. This paper is an attempt to set out a positive theory of the doctrine on preemption of products liability claims. The case law is largely consistent with an approach that seeks minimize the costs of erroneous decisions to preempt tort lawsuits. In particular, two factors explain many of the outcomes of the preemption cases: agency independence and the degree of congruence between the regulatory and common law standards.