Beyond a reasonable doubt is the highest standard of proof in the legal system. It applies in criminal trials. The prosecution must convince the jury that no reasonable explanation exists other than the defendant's guilt. If a reasonable doubt remains, the jury must acquit.

The standard exists to protect the innocent. Blackstone's ratio, often quoted, says it is better that ten guilty persons escape than one innocent suffer. The US Constitution requires this level of proof for criminal convictions. It does not require absolute certainty, which is rarely possible. It requires a firm conviction based on reason and evidence.

Standards of proof compared

• Mere suspicion: no evidence, just a hunch.
• Reasonable suspicion: specific facts justifying a brief stop.
• Probable cause: enough evidence for an arrest or warrant.
• Preponderance of the evidence: more likely than not, used in civil cases.
• Beyond a reasonable doubt: near certainty, used in criminal cases.

Jurors often struggle with the phrase. Courts discourage attempts to quantify it as a percentage. It is not 95 percent or 99 percent. It is a level of certainty that leaves no reasonable doubt in the mind of a reasonable person. Defense attorneys attack the evidence. Prosecutors carry the burden. If the burden is not met, the defendant walks free, even if the jury suspects guilt.