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 California, evidence is regulated by both state statutes and case law, which dictate what can be admitted in court during a lawsuit. Evidence can be in the form of physical objects, digital data, written documents, or witness testimony. For evidence to be admissible, it must be relevant to the case, meaning it must have a tendency to prove or disprove a disputed fact that is of consequence to the determination of the action. California follows the California Evidence Code, which outlines the rules for admissibility, including the exclusion of evidence that is not relevant or is considered hearsay, unless an exception applies. Expert witness testimony is subject to particular scrutiny; it must be provided by someone qualified through expertise, and the testimony must be based on matter that is sufficiently beyond common experience that the opinion of an expert would assist the trier of fact. The reliability of expert testimony is assessed under the standards set by the California Supreme Court in cases such as Sargon Enterprises, Inc. v. University of Southern California. The judge acts as the gatekeeper in determining the admissibility of evidence, including the reliability of expert testimony.