The Montrose doctrine is a legal principle, enunciated by the California Supreme Court in Montrose Chem. Corp. v. Admiral Ins. Co., 10 Cal. 4th 645, 42 Cal. Rptr. 2d 324, 913 P.2d 878 (1995).
The ruling held that injury or damage for which an insured may incur liability is not a "known loss"—hence uninsurable under basic precepts of insurance law—until liability for the injury or damage has been assessed by a court. As a result of Montrose, standard general liability policies now exclude liability from any injury or damage known to the insured at the time the policy takes effect.