The law of evidence regulates the tendering and evaluation of the various forms of proofs at the trial. It is designed to facilitate the discovery of truth. A court decides on the admissibility of any evidence in trial according to the rules of evidence.
Before the promulgation of the Federal Rules of Evidence in 1975, US Evidence Law was largely a creature of the common law traditions.
In the US while the federal courts follow the Federal Rules of Evidence, different state courts apply their own state rules made by the concerned state legislatures. The Federal Rules of Evidence consists of 68 sections. Many of the state codes of evidence have borrowed profusely from the Federal Rules of Evidence and contain similar provisions. In both civil and criminal proceedings the Federal Rules of Evidence apply in all federal courts.
For determining what evidence is admissible, the first and foremost parameter is the relevancy of the offered evidence in the trial. The rules of evidence also allocate among the parties the burden of proof of different propositions. Such rules also decide upon the admissibility of hearsay and oral testimony.