This chapter shows how the principles of environmental integration and precaution emerged in European Union (EU) law and how they are understood and interpreted by EU institutions, scholars and EU Courts, through the examination of EU Treaties and the most important case law of the European Court of Justice and the General Court. The case law of the European Courts involving the application of the precautionary principle can be grouped around cases dealing with environmental issues, and around cases that concern health and food safety issues. The integration principle is considered as general principle that 'serves as the very backbone' of concept of sustainable development. The integration principle is aimed at not only by policy declarations but by several international treaties and agreements. The concept of integration appeared for the first time in the First Environmental Action programme of the European Community, although it was not listed as one of the 11 principles of its newly established environmental policy.