Legal presumptions serve essential and legitimate needs in national legal orders, where they are introduced for the purpose of simplifying the assessment and the recovery of tax or for tackling tax avoidance or evasion. Nowadays, however, the national choices concerning the use of tax law presumptions must be tested against European Union (EU) law, both in uniform or harmonized sectors and, albeit to a different extent, in non-harmonized ones. In this article, after a brief discussion of the meaning and relevance of the concept on the basis of national juridical traditions, the case law of the Court of Justice of the European Union (CJEU) concerning the repayment of taxes unduly levied is analysed, so as to highlight the core scheme adopted when it evaluates the national tax law presumption's compatibility with EU law. Afterwards, an examination of the most relevant CJEU case law on legal presumptions in different tax matters (customs law, VAT, direct taxation) is carried out. This, with a view to catching some interpretative trends peculiar to specific domains of taxation which should guide national legislators. Finally, some general conclusions are developed on the overall approach to national tax law presumptions in the context of EU law, with an eye to national experiences.