Burden of proof refers to the amount of evidence or proof required to adequately prove disputed facts in a court of law, and which party has the obligation to prove the disputed facts in order to prevail in court. In criminal prosecutions (cases) the government or prosecution has the burden to prove the accused defendant’s guilt beyond a reasonable doubt. This has been the standard for criminal prosecutions in the United States since at least 1880 when it was first mentioned by the U.S. Supreme Court. The determination of whether there is reasonable doubt as to an accused defendant’s guilt may be characterized by the question of whether there is an alternative explanation of the facts that seems plausible to the jurors.

In Arizona, as in all states across the United States, the burden of proof in criminal prosecutions lies with the government or prosecution. They must prove the defendant's guilt 'beyond a reasonable doubt,' which is the highest standard of proof required in the American legal system. This standard has been a cornerstone of criminal law since it was articulated by the U.S. Supreme Court in the 1880s. It ensures that a defendant's liberty is not taken away unless the evidence presented by the prosecution is so convincing that there is no reasonable doubt left in the minds of the jurors regarding the defendant's guilt. If an alternative explanation of the facts exists that seems plausible to the jurors, then the prosecution has not met its burden, and the defendant should not be convicted.