In this asbestosis case, plaintiffs added Air Brake in place of a Doe defendant shortly after Boeing produced documents showing that Air Brake designed the brake shoes for its light rail cars, specifying brake pads containing asbestos. The trial court erred in granting summary judgment on limitations grounds after concluding that plaintiffs “knew or should have known” of Air Brake’s existence when they filed the complaint. The correct test under CCP 474 is whether plaintiffs actually knew facts supporting a claim against the party later named as a Doe Defendant. Ignorance of those facts allows naming the party as a Doe Defendant even if that ignorance is a result of the plaintiffs’ negligence.