With its judgment of 10 September 2019, the General Court (the “Court”) annulled an earlier 2 Commission decision to modify the exemption regime for the OPAL pipeline. The Court based its judgment on the principle of energy solidarity introduced by Article 194(1) of the Treaty on the Functioning of the European Union (“TFEU”). The ruling goes against the old adage that only small cases make great law, as the OPAL pipeline exemption has been a major bone of contention in EU energy policy for many years. Given the role and history of legal principles in European Union (“EU”) law, the judgment may have far reaching impact on the interpretation and application of the rules of EU energy law. This article will focus on one aspect of the OPAL case: the principle of energy solidarity.