This chapter examines the principle of the legality of taxation in the contextof the rule of law. It begins with an exploration of the legality of taxation ininternational and EU Law, providing a comparative overview and delvinginto the specific case of Sweden. The authors discuss how this principleis not explicitly stated as a fundamental right in the European Charter ofHuman Rights and Fundamental Freedoms but can be derived from itsprovisions. The Court of Justice of the European Union recognizes it as ageneral principle, necessitating that tax obligations and their substantiveelements be defined by law.The article then focuses on the Swedish interpretation of the principle,tracing its evolution and current application. It highlights the nuances in theSwedish legal system, where the principle of legality in tax law is interpretedas a general rule that taxation must be lawful and in accordance with theprinciples of the rule of law. This includes a discussion of the role of theSwedish Parliament in enacting tax laws and the interpretative approachesof the Supreme Administrative Court.In conclusion, the paper emphasizes the principle of legality as an integralpart of a larger legal system grounded in the rule of law. It addresses thechallenges and complexities in applying this principle, especially whenharmonizing national and EU law principles in tax cases.