Under Ins. Code 1861.03(c)(1), an insurer may cancel an auto insurance policy if there is a substantial increase in the insured risk. Insurance Department regulations define substantial increase to include the insured’s failure to respond within 30 days to the insurer’s reasonable written request for information necessary to accurately underwrite or classify the risk. Here, the trial court correctly granted the insurer summary judgment against a claim under an auto insurance policy which the insurer had canceled before the accident due to the insured’s failure to answer the insurer’s written inquiry whether the insured wanted his driving age son to be included as an insured under the policy. That information was reasonably necessary to underwrite the risk, and the insured’s failure to answer the question justified the cancellation of the policy.