The paper deals with the joint investigation teams in the EU that are considered as a necessary tool to enhance the internal security of the EU. They are the desired instruments for international cooperation of judicial and law enforcement authorities in the fight against organized cross-border crimes as well as a useful tool in facilitating mutual assistance in criminal matters between the Member States of the EU. Joint investigation teams have demonstrated their usefulness in investigating the most serious forms of criminality such as terrorism or drug trafficking. The paper firstly focuses on the development of joint investigation teams and their legal basis at the level of the EU, namely the Convention on mutual assistance in criminal matters between the Member States of the EU and the Framework Decision on joint investigation teams. Further, it introduces the establishment of joint investigation teams. JITs are established on the basis of a written mutual agreement between two or more Member States of the EU. Special attention is focused on national experts of joint investigation teams and the participation of Europol and Eurojust in joint investigation teams. Both Europol and Eurojust were established to support Member States of the EU in the fight against serious organized cross-border crime. Both of them can participate in JITs separately as well as jointly. Moreover, it discusses their implementation in the European Union legal system, as well as common problems.