A legal rule that can delay the start of a filing deadline until an injured person knew, or reasonably should have known, that they were harmed and that the harm may have been caused by someone else's conduct.

That matters when an injury is not obvious on day one. A worker may brush off back pain after a fall, only to learn months later that a disc injury is serious; a patient may not discover a surgical error until follow-up treatment; someone exposed to a toxic substance may not connect symptoms to the exposure right away. The statute of limitations usually starts when a claim "accrues," and the discovery rule can shift that date. Courts still look at reasonableness, though, so waiting forever is not an option just because the body takes its time delivering bad news.

In Massachusetts, most personal injury claims must be filed within 3 years under G.L. c. 260, § 2A. The discovery rule can extend when that 3-year clock begins, but only if the facts support delayed discovery. In an injury claim, that can decide whether a case is heard or dismissed before it gets anywhere. It also affects evidence: medical records, work logs, and early complaints may help show when the injury became knowable, which is often where the real fight starts.