Quorum requirements for a referendum to be valid

Montenegro
Answer
- Mandatory referendum - turnout quorum
- Optional referendum - turnout quorum
Source
Law on Referendum (2001)
http://www.legislationline.org/documents/id/3934
Article 37
Thedecision in a referendum is taken by a majority vote of the citizens who havevoted, provided that the majority of citizens with voting rights has voted.
Comment
The results of the referendum are valid if majority of citizens with voting rights have voted in the referendum.