This paper explores the conditions under which a criminal law may be said to be arbitrary and, thus, constitutionally infirm. A finding of arbitrariness is a profound condemnation. Though the doctrine has re-emerged in recent Canadian jurisprudence, it remains largely unexplored. Some cases purport to apply a stringent test, requiring that the law fail completely to establish a logical link between, say, its goals and means. Other cases hint at a broader approach. The broader model can be satisfied in a number of situations: where a law is radically under-inclusive; where it fails to establish that particular harms are sufficiently distinct to warrant different legal treatment; or where it betrays illegitimate state goals. All of those situations present fatal challenges to validity. Recognizing them would develop a concept of arbitrariness that better accords with fundamental justice and respect for individual liberty.