Objectives:
The subject of the study is the solidarity clause introduced into the EU law system by the Treaty of Lisbon. Currently, it is in the article 222 the Treaty on the Functioning of the European Union. The aim of the study is to show its history, legal basis in the treaty regulations and in the secondary law and the way it operates. The work also shows the concerns and obstacles in its application.

Material and methods:
The method of dogmatic analysis of the law and the descriptive method were applied in the work.

Results:
Particularly, the attention has been paid to its possible conflict with the concept of the sovereignty of the Member States. As a research hypothesis, the assumption has been made that the solidarity clause is a great opportunity for greater European integration by cooperating in the situations of threat to the security of the society in one of the Member States.

Conclusions:
As a final conclusion, it should be assumed that the solidarity clause is an important instrument of the Union's security policy.