Persons moving within the Community are subject to the social security scheme of only one single Member State, generally of this in which the person concerned pursues his activity. However, this solution does not meet the expectations of mobile workers and their employers in many cases. Article 16(1) of Regulation No 883/2004 provides for the possibility of determining a different social and legal status by concluding an agreement. The subject of this paper is to identify and discuss legal problems relating to: 1) entities entitled to submit an application for the conclusion of an agreement, 2) "interest of certain persons or categories of persons" as a premise allowing its conclusion, 3) its parties and consequently also its legal nature, and 4) legal remedies available to interested parties in the event of a refusal to conclude it.