The rule by which an original writing must be offered as evidence unless it is unavailable.

A rule of evidence that demands that the original of any document, photograph or recording be used as evidence at trial, rather than a copy. A copy will be allowed into evidence only if the original is unavailable.

The best evidence rule is a common law rule of evidence which can be traced back at least as far as the 18th century. In Omychund v Barker (1745) 1 Atk, 21, 49; 26 ER 15, 33, Lord Harwicke stated that no evidence was admissible unless it was "the best that the nature of the case will allow". The general rule is that secondary evidence, such as a copy or facsimile, will be not admissible if an original document is available.