Evidence is information that exists in physical or digital form—such as written agreements and e-mail communications—as well as testimony from witnesses who have personal knowledge of facts relevant to a dispute. Evidence generally must be relevant to be admitted in court and considered by the factfinder (judge or jury) in resolving a lawsuit. And some forms of evidence, such as expert witness testimony, must be shown to be sufficiently reliable (as determined by the judge) to be admitted in evidence.

In Arizona, evidence is crucial in both civil and criminal proceedings and must adhere to the rules of evidence as outlined by the Arizona Rules of Evidence. These rules determine what evidence is admissible in court. For evidence to be admitted, it must be relevant, meaning it has any tendency to make a fact more or less probable than it would be without the evidence and the fact is of consequence in determining the action. Physical and digital evidence, such as written agreements and email communications, must be authenticated to be admissible. Witness testimony, including that from expert witnesses, must be based on sufficient facts or data and be the product of reliable principles and methods that the witness has applied reliably to the facts of the case. The judge serves as the gatekeeper in determining the admissibility of evidence, including the reliability of expert witness testimony. The Arizona Rules of Evidence are consistent with the Federal Rules of Evidence, ensuring a degree of uniformity with federal proceedings.