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 Idaho, as in other states, evidence must be relevant to be admissible in court. Relevance is determined by whether the evidence makes a fact more or less probable than it would be without the evidence and whether the fact is of consequence in determining the action. Both physical and digital forms of evidence, such as written agreements and email communications, can be admitted as long as they meet the relevance criteria. Additionally, witness testimony is admissible if the witness has personal knowledge of the facts at issue. Expert witness testimony is subject to a reliability standard, which is assessed by the judge. The judge must determine that the expert's testimony is based on sufficient facts or data, is the product of reliable principles and methods, and that the expert has applied the principles and methods reliably to the facts of the case. This is in line with the federal Daubert standard, which Idaho courts have adopted for determining the admissibility of expert testimony. The rules governing the admissibility of evidence in Idaho are primarily found in the Idaho Rules of Evidence.