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 Louisiana, evidence is regulated by both state statutes and the Louisiana Code of Evidence. Evidence must be relevant to the issues in a legal dispute 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 that fact is of consequence in determining the action. Physical and digital evidence, such as written agreements and email communications, must adhere to rules regarding authenticity, reliability, and the absence of hearsay, unless an exception applies. Testimony from witnesses with personal knowledge of relevant facts is also considered evidence. Expert witness testimony, in particular, must meet a standard of reliability. In Louisiana, the judge acts as the 'gatekeeper' to determine the admissibility of expert testimony, ensuring that it 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 Daubert standard, which is a common federal standard for the admissibility of expert witnesses' testimony, and has been adopted in Louisiana as well.