Universi Dominici Gregis is an Apostolic Constitution of the Catholic Church issued by Pope John Paul II on February 22, 1996. It superseded Pope Paul VI's 1975 Apostolic Constitution, Romano Pontifici Eligendo.
Universi Dominici Gregis ('the Lord's whole flock', from the opening statement 'The Shepherd of the Lord's whole flock is the Bishop of the Church of Rome, ...'), subtitled On the Vacancy of the Apostolic See and the Election of the Roman Pontiff, deals with the vacancy of the Chair of St Peter and Bishop of Rome namely the Pope.
The constitution modified or in some cases confirmed the rules, for the conclave. It also clarified, during a Sede Vacante, which matters could be handled by the College of Cardinals and which matters were reserved for the future Pope.
Read more about Universi Dominici Gregis: Secrecy, Previous Methods of Election, Living Quarters of Cardinals, Major Changes, 2005 Papal Conclave, Changes