一事不再理是普世基本法律原則。不管在國際法或內國法層次，一事不再理都只針對同一法秩序內部之禁止重複追訴而設。惟在今日歐盟，一事不再理原則正從內國效力走向跨國效力。歐盟跨國一事不再理之規範基礎有二：《申根施行公約》第 54 條和《歐盟基本權利憲章》第 50 條。兩者的主要差異是執行要件，這正是歐盟法院 Spasic 裁判的關鍵。歐盟法院認為，《申根施行公約》第 54 條是《歐盟基本權利憲章》第 50 條一事不再理權利之限制；《申根施行公約》第 54 條規定，適用一事不再理原則時，有罪確定判決應受到執行要件之拘束，亦即，必須制裁「已執行完畢」或「正在執行中」，這與保障一事不再理原則之《歐盟基本權利憲章》第 50 條相符。據此，《歐盟基本權利憲章》第 50 條所保障之一事不再理原則，應以《申根施行公約》第 54 條執行要件為適用限制。

“Ne bis in idem”, or double jeopardy, is a universal principle. Whether in international or domestic law, the principle of double jeopardy is restricted within the legal system of a single nation. However, the European Union is expanding the effect of double jeopardy from domestic to transnational. The supranational double jeopardy principle in the European Union is based on 1) Article 54 of the Convention implementing the Schengen Agreement, and 2) Article 50 of the Charter of Fundamental Rights of the European Union. These two mainly differ in their “enforcement”, which is the key to the Spasic judgment of the Court of Justice of the European Union. According to the Court, Article 54 of the Convention implementing the Schengen Agreement is the restriction of the double jeopardy principle stated in Article 50 of the Charter of Fundamental Rights of the European Union. Article 54 of the Convention implementing the Schengen Agreement states that, when double jeopardy is applied, a criminal conviction must be limited by the enforcement requirement. In other words, a sanction must be “completed in enforcement” or “enforcement in progress”, which aligns with the double jeopardy prince detailed in Article 50 of the Charter of Fundamental Rights of the European Union. Based on this idea, the double jeopardy principle, under the protection of Article 50 of the Charter of Fundamental Rights of the European Union, should utilize Article 54 of the Convention implementing the Schengen Agreement’s enforcement requirement as its applicable limitation.