Istanbul Convention is one of the first legislative acts of the Council of Europe that addressed the problem of gender-based violence. It has, however, since its beginnings, been controversial. It also serves as a notable illustration of a mixed agreement in EU law, since its scope falls both within the Member States’ competences and those of the EU. In 2017, the EU as an international actor signed the Convention, thereby signaling its intent to develop a more comprehensive legislative framework on the issue of gender-based violence. However, many Member States expressed their opposition when the EU started to consider the next step of the accession to the Convention – its ratification. Therefore, the European Parliament has decided to ask the Court of Justice of the EU for a formal Opinion on the procedure of the EU accession to the Convention. In the Opinion 1/19, the Court of Justice has, in principle, given a clear green light to the EU’s ratification of the Istanbul Convention by delivering a ruling according to which mixed agreements can be ratified by the EU without a common accord of its Member States. Following the delivery of the Opinion 1/19, the EU ratified the Convention in 2023. This is why the Opinion may have significant impact in the future, particularly in the development of EU’s capacity to act in an international context. This thesis aims to describe the characteristics of mixed agreements in EU law and the main features of the Istanbul Convention, analyze Opinion 1/19 of the Court, as well as to outline the key legal implications of the recent EU’s ratification of the Convention.