In Canadian public law, the foundational case of Roncarelli v. Duplessis stands for the proposition that arbitrariness and the rule of law are conceptually antithetical values. This article examines multiple forms of arbitrariness in Roncarelli, going beyond the usual focus on discretionary power arbitrarily exercised by the executive branch of government. A close reading of the case not only brings to the surface other forms of arbitrariness, notably under-acknowledged forms of judicial arbitrariness, but also illuminates how legal actors attempt to constrain arbitrariness within the activity of judging. Furthermore, repositioning the case in its larger social and political context provides an alternative vantage point from which the core conceptual content can be enlarged and the case’s normative import better gleaned. Indeed, what is most surprising about Roncarelli is how well it tracks the meanings and attributes of arbitrariness identified by legal theorists such as Joseph Raz, Henry Richardson, Jeremy Waldron and others. Legal theory and practice both confirm reason-giving as one significant rule of law practice that constrains arbitrariness by seeking to ensure that decision-makers throughout the state are attuned to the demands of legality, can be publicly held to account, and are committed to upholding principles of good government.