This chapter concerns two key concepts of environmental law: environmental principles and environmental policy. Both concepts are well known to those who study and practise UK and EU law, but that familiarity can be deceiving when it comes to understanding their role in environmental law, because both principles and policy perform important, distinctive, and evolving functions. Environmental principles are highly symbolic ideas of environmental policy that have been developing prominent roles in environmental law globally, including in EU environmental law. Environmental policy is often implicated in environmental law regimes because of the need to respond quickly to changing circumstances and provide detailed and technical guidance in complex policy areas. Determining the legal implications of extensive reliance on policy in environmental law is thus important. Exploring both these distinctive legal features of environmental law—principle and policy—helps to elucidate different aspects of environmental law as a subject, interrogating the jurisprudential nature of environmental law and revealing key characteristics of its developing doctrine.