Coverage on your own policy that pays when the driver who hurt you had no insurance, or nowhere near enough - and it is the coverage people most often discover they declined.
Uninsured motorist coverage applies when the at-fault driver had no liability insurance, or cannot be identified at all, as in a hit-and-run. Underinsured motorist coverage applies to the much more common situation where the other driver had insurance but the policy limit is smaller than the harm done. Both sit on the injured person's own policy, and both are the reason a serious injury caused by a minimally insured driver is not automatically uncompensated.
Because the claim is made against one's own insurer, it has a different shape from a claim against a stranger. The insurer is simultaneously the party that owes the contract and the party disputing the amount, and there are usually notice requirements, cooperation duties and consent requirements - in particular, settling with the at-fault driver without the insurer's consent can forfeit the underinsured claim entirely.
Whether the coverage must be offered, whether it can be declined and how, and whether limits from several vehicles or policies can be combined are all state questions, and the answers vary widely.
Two moments matter. The first is before any settlement with the at-fault driver, because consent requirements are routinely missed and the forfeiture they cause is not reversible. The second is when an own-insurer claim is denied or valued far below the harm, since a dispute with one's own insurer can raise contractual and bad-faith questions that a straightforward injury claim does not.
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