Despite its largely preventive orientation, environmental law has, with one exception, remained distinct from the burgeoning field of preventive justice. The exception is the precautionary principle, which has become a subject of interest and frequent skepticism amongst preventive justice scholars. The precautionary principle is a central principle in environmental law. Its centrality arises from the pervasiveness of scientific uncertainty in environmental regulation; that is, our inability to reliably predict the consequences of our policy choices on environmental and human health. The precautionary principle squarely addresses the question of how we ought to proceed in the face of unavoidable uncertainty. This chapter explores the connection between the precautionary principle in environmental law and preventive justice scholarship. It is written from the perspective of environmental law. It has two aims. First, the chapter traces the claims about the precautionary principle in the preventive justice literature back to their environmental roots. In doing so, it situates the precautionary principle within broader environmental law debates about how the principle ought to operate. Second, the chapter argues that the precautionary principle and principles of preventive justice ought to be understood as part of the same rule-of-law project, that is, the project of ensuring that all public decisions are publicly justified on the basis of core constitutional principles. Understood in this way, both the precautionary principle and principles of preventive justice seek to respect and enable the autonomy of individuals, understood as their capacity to reason with the law. It is thus a mistake to think that the precautionary principle can be transposed from the environmental context to the national security context. What the rule of law requires in each context is distinct and these respective principles ought to be understood as attempts to maintain our commitment to the rule of law even under the challenging conditions in which we face uncertain future harm.