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 Montana, as in other states, evidence must be relevant and reliable to be admissible in court. Relevance means that the evidence must have some bearing on the facts of the case or be helpful in resolving the issues at dispute. Reliability pertains to the quality of the evidence, which is particularly scrutinized when it comes to expert witness testimony. Montana follows the rules of evidence which are similar to the Federal Rules of Evidence. Expert witnesses, for example, must be qualified by knowledge, skill, experience, training, or education, and their testimony must be based on sufficient facts or data and be the product of reliable principles and methods. The judge serves as the gatekeeper in determining the admissibility of evidence, including the reliability of expert testimony. Physical and digital evidence, such as written agreements and email communications, must also meet the criteria of being relevant and not excluded by other rules, such as hearsay or privilege, to be considered by the judge or jury.