Of all the issues negotiated at Nice, changes to the Commission were among the most difficult to agree.
The Commission develops proposals for EU laws and policies and acts as guardian of treaties. Although any new laws must be approved by the member-states meeting in the Council of Ministers (and sometimes by the European Parliament), it is the Commission's job to implement and enforce them.
Since the creation of a single market, the Commission's role as the EU's competition authority has become increasingly important. Although its members are appointed by the member-states, the Commission acts on behalf of the EU as a whole.
At present, each the biggest five member-states - Germany, France, Britain, Italy and Spain - appoints two members of the Commission and the others appoint one Commissioner each. The President is chosen unanimously by the heads of government of the 15 member-states.
In Amsterdam four years ago, the big countries agreed to give up their second Commissioner if a wider package of reforms was agreed to prepare the EU to accept new members. This will take effect when the next Commission takes office in 2005.
Under the Nice Treaty, each member-state would nominate one Commissioner until the EU grows to include 27 countries. From then on, a rotation system will operate so that member-states take turns to lose their Commissioner for a five-year term. The precise nature of the rotation system has yet to be agreed but it will operate on the basis of strict equality so even the biggest countries will have to take their turn.
The Commission President will no longer be chosen unanimously but by qualified majority voting. This will mean a country cannot veto a nominee who is the choice of almost everyone else. The President will become more powerful, able to move Commissioners to different portfolios and, in some cases, to sack them.
The role of the Commission is one of the issues to be discussed in the formal debate about the EU's future shape which was launched at the Nice summit.