This chapter will examine the most important principles of environmental law, namely: sustainable development (or sustainability); the social and environmental function of property; polluter-pays principle; prevention and precautionary principle; publicity; public participation; non-environmental regression; and in dubio pro natura (in doubt, in favor of nature). The doctrinal and legal contours of these principles will be analyzed, as well as their application in concrete cases by the courts. In Brazil, legal principles hold particular importance in judicial reasoning, as judges and courts can impose implicit duties on public authorities and private individuals based on constitutional provisions. These principles establish duties, especially concerning environmental protection, and assist the courts in interpreting legislative acts.