Underinsured Motorist coverage (UIM) is first-party auto insurance that allows injured motorists to file a claim against their own policy if an at-fault driver does not have sufficient insurance to cover the damages caused in an accident. Uninsured Motorist coverage (UM), on the other hand, is first-party insurance coverage that allows policyholders to file a claim against their own policy when an at-fault driver is uninsured or completely unidentified, such as in the case of a hit-and-run driver.