Abstract This chapter examines the role of commonly espoused environmental principles in US and Canadian law. A number of substantive principles are evident in the environmental laws of both countries, including: cost-benefit analysis, transboundary responsibility, sustainable use, sustainable development, pollution prevention, precaution, integration, polluter pays, strict liability, and extended producer responsibility. However, they often play a secondary role to political and economic considerations in shaping environmental policy. Thus, the positive functions that substantive principles are sometimes thought to perform – increasing coherence, legitimacy, transboundary consistency, and potentially the progressive realization of environmental protection – appear relatively muted in the US and Canadian legal systems. More ‘procedural’ principles, such as federalism, public participation, public information, availability of remedies, judicial review, and private enforcement, however, are well rooted and broadly important in both systems. Substantive principles may play a more significant role in local government and non-state environmental governance institutions.