The thesis is a comparative legal analysis of environmental principles in environmental law. Environmental principles are novel concepts in environmental law and they have a high profile in environmental law scholarship. This high profile is promoted by two factors – the high hopes that environmental law scholars have for environmental principles, and the increasing prevalence of environmental principles in legal systems, particularly in case law. This thesis analyses the latter, mapping doctrinal developments involving environmental principles in two jurisdictions and court systems – the courts of the European Union and the New South Wales Land and Environment Court. This doctrinal mapping has both narrow and broad aims. Narrowly, it identifies the legal roles in fact taken on by environmental principles within legal systems. Broadly, and building on this assessment, it responds to scholarly hopes that environmental principles (can) perform a range of significant roles in environmental law, including solving both environmental problems and legal problems in environmental law scholarship. These hopes are based on assumptions about environmental principles that have methodological weaknesses, including that environmental principles are universal and that they fit pre-existing models of ‘legal principles’ drawn from other areas of legal scholarship. The thesis exposes these methodological problems and concludes that environmental principles are not panaceas for pressing and perceived problems in environmental law. It does this by showing that the legal roles of environmental principles, which are significant in environmental law and its current evolution, can only be understood by closely analysing the legal cultures in which they feature. This is a conclusion for environmental law scholarship generally – while environmental issues and problems may be urgent and often global, legal analysis of the law that applies to those problems requires close engagement with legal systems and cultures, as they are and as they develop.