A written statement of facts that a person swears or affirms is true, usually signed before a notary or other authorized official.

Affidavits are used to put key facts into a form a court, insurer, or agency can rely on without hearing live testimony right away. A witness might give one describing what they saw after a crash, a doctor might provide one about treatment and limitations, or a records custodian might use one to authenticate documents. In civil cases, affidavits often appear with motions, including requests for summary judgment, where one side argues there is no real dispute requiring a trial.

In an injury claim, the value of an affidavit depends on how specific, accurate, and firsthand it is. A clear affidavit can help establish liability, support damages, explain lost wages, or preserve a witness's account while memories are still fresh. For example, after a collision in dense fog on Cape Cod, an affidavit from a driver, passenger, or responding witness may help pin down visibility, speed, and road conditions.

Massachusetts practice gives affidavits a practical role in litigation, especially under Massachusetts Rule of Civil Procedure 56, which governs affidavits used in summary judgment proceedings. A false affidavit can seriously damage credibility and expose the signer to penalties for perjury, so lawyers usually review wording carefully before filing one with the court.