一事不再理原則，又稱一罪不受兩次審理原則，幾乎是所有國家的刑事程序的基本原則，且通常是受憲法所保障的人權，係指人民同一犯罪行為不受重複之追訴。一事不再理原則包含雙重層面之意旨，其一為保護經實體判決確定之被告，免於再接受一次訴訟程序的騷擾、折磨之目的；其二旨在維護法安定性，保障任何經判決有罪或無罪開釋確定者，無庸就同一行為再受一次刑事訴究。 傳統上，大多數國家僅將一事不再理原則應用於內國刑法的範圍；然而在歐盟中，一事不再理原則已經擴張至跨境領域。一事不再理原則原本僅具有拘束內國法院之效力，隨著跨境犯罪的增加及國際刑事司法合作進一步開展，一事不再理原則已有非主權化的趨勢，原本作為一項解決司法管轄權積極衝突的普遍性原則，一事不再理原則現已儼然是應適用於國際的或者跨國層面的刑事基本權利。 本文主要是以申根施行公約及歐盟基本權利憲章規定為論述主軸，特別著重於一事不再理原則在歐盟刑法統整中之演變發展過程。此外，本文的重點將關注一事不再理原則在我國目前國際刑事司法互助上之最新發展。

Ne bis in idem is a fundamental legal principle common to practically all national criminal justice orders , usually as a constitutional human right It is also known as the prohibition of double jeopardy. According to this principle, a person can not be prosecuted more than once for the same criminal behaviour. Its ratio is twofold : on the one hand, to offer judicial protection to persons against the State’s jus puniendi, once they have been subject to a prosecution, and on the other hand to ensure legal certainty and the respect of the res judiciata. Traditionally, in most States ne bis in idem applied solely at national level. However, within the EU this principle has been shifted to the transnational level. The principle has also been laid down in Article 50 of the Charter of Fundamental Rights of the European Union. Since the EU has developed this principle from a domestic to a transnational legal principle and fundamental right, his development can be explained by a specific link between ne bis in idem and the freedom of movement within the EU. Moreover, the increasing role played by judicial cooperation has also lead to the principle becoming de-sovereignize. The study will focus on Elements in art.54 art.54 of the Convention Implementing the Schengen Agreement and art. 50 of the Charter of Fundamental Rights of the European Union, especially on relation of both European Union law for application of the principle Ne bis in idem. Besides recent development of this principle in taiwan’s law, including treaty or agreement with other countries will be also a central point of this essay.