Primary evidence and secondary evidence are categories used to describe the nature of a piece of evidence. While primary evidence is preferred, secondary evidence is acceptable in some circumstances.

FindLaw for Legal Professionals defines evidence as "something that furnishes or tends to furnish proof." The judge or jury uses evidence to figure out what really happened in a case.

According to West's Encyclopedia of American Law, primary evidence is "an authentic document or item that is offered as proof in a lawsuit, as contrasted with a copy of, or substitute for, the original."

West's says, "secondary evidence is evidence that has been reproduced from an original document or substituted for an original item." A photocopy of an original document and a model of a crime scene are examples of secondary evidence.

Primary evidence is sometimes called "best evidence." The Federal Rules of Evidence require an original to prove the content of a document, photograph or recording, but will allow a copy in some circumstances. This requirement is called the best evidence rule.

Historically, courts considered duplicates of documents unreliable because they were copied by hand. After modern technology improved the reliability of copies and models, courts began to regularly allow secondary evidence.