One of the conditions for a detention not to be arbitrary under s. 9 of the Canadian Charter of Rights and Freedom is that it must be authorized by a law that is itself not arbitrary. The Supreme Court’s pronouncements about what makes a law arbitrary have been brief, question begging and, at times, conflicting. This article takes stock and proposes a path forward. First, it reviews the Supreme Court’s case law with the goal of developing a taxonomy of its various accounts of arbitrary laws. It identifies three: a formal account (with a procedural check), a purpose-sensitive account, and a discrimination/improper considerations account. It then evaluates these accounts in light of the purpose of s. 9, which is to ensure that detentions are adequately justified from the perspective of the ideal of the rule of law. It argues that, when properly interpreted, the Court’s three accounts correspond to different aspects of this ideal. As such, they should be applied cumulatively, and continue to be developed along that purposive axis. Finally, the article considers the Supreme Court’s use of the ancillary powers doctrine to create new powers of detention at common law. It argues that this doctrine, which currently evades direct section 9 scrutiny, should also incorporate a distinct assessment of arbitrariness along the lines identified—especially since, properly understood, it is itself a doctrine aimed at advancing the rule of law. Overall, the article suggests that the key parameters for a consistent and principled approach to what makes laws arbitrary are already present in the Supreme Court’s s. 9 case law (and related aspects of its ss. 7 and 15 case law). What is needed is for the Court to organize them systematically based on the purpose of s. 9, and to commit to advancing them, whenever applicable, through its evolving interpretation this section.