In this chapter I discuss the role of the presumption of constitutionality in Canadian constitutional law. I describe the presumption’s origins and explain how it has been applied by courts since the coming into force of the Canadian Charter of Rights and Freedoms. I then situate the presumption relative to other interpretative techniques courts employ to uphold legislation as constitutional, a practice which reached a high-water mark in the Supreme Court of Canada’s decision in Canadian Foundation for Children, Youth and the Law v Canada (Attorney General). In that case, a majority of the Court adopted what can only be described as an implausible interpretation of the so-called “spanking defence” in the Criminal Code before upholding it as constitutional. I consider what might motivate courts to rely on the presumption in borderline cases. I also offer some thoughts on the dynamics of legislating in the shadow of the presumption.