On its tenth anniversary, a great deal can be said about the decision in Dunsmuir v New Brunswick and the way that it and its progeny have impacted how courts approach judicial review in Canada. The grounds upon which decisions may be quashed have expanded, and reviewing courts now must consider the facts and circumstances underlying a decision under review as well as their impact on the Charter interests of a party challenging a decision. Nevertheless, the rules governing evidence admissible on judicial review have not been changed in any meaningful way since the nineteenth century. Those rules, generally speaking, still limit the admissibility of so-called ‘extrinsic’ evidence to situations involving alleged ‘jurisdictional errors’ or a breach of the duty of fairness. While perhaps not as exciting as other current issues in administrative law, it is one of tremendous practical significance to parties seeking to challenge the decisions of public authorities. The rigid evidentiary rules can mean that parties are not practically able to make the arguments that they would otherwise be entitled to make on review, and the task of the reviewing court can be frustrated. Operating from the thesis that the rules of evidence on judicial review should move in tandem with changes to the substantive law of judicial review, the authors propose a general rule permitting parties to adduce evidence necessary to the arguments they are permitted to make, subject to the courts’ discretion to exclude such evidence where there is a meaningful opportunity or expectation that the evidence be put before the decision maker at first instance. The issue of ‘what is the record’ is increasingly a live one on judicial review, particularly when non-adjudicative or policy decisions are under review. This is unlikely to change when the Supreme Court of Canada revisits Dunsmuir, unless the rules of evidence are modified in tandem.