By Laurence Peter
Will Mr Van Rompuy (left) and Mr Zapatero avoid rivalry in EU affairs?
From 1 January Spain takes the helm of the EU - a role which will define how member states work with the new EU president and foreign policy chief.
Spain's six-month term as holder of the EU presidency comes just a month after the Lisbon Treaty took effect.
EU summits will be chaired by Herman Van Rompuy, the former Belgian prime minister who is now President of the European Council.
But Spain will steer other top meetings and host summits with non-EU countries.
Critics say the Lisbon Treaty allows too much scope for overlapping roles, as the six-month rotating presidency continues, potentially allowing a member state to further its own agenda.
A key aim of Lisbon is to develop continuity in EU policy-making and reduce the disruption caused by the six-month rotation.
Spain will chair important EU ministerial meetings on the economy, eurozone policy, the environment and energy, as well as other sectors.
A major EU goal in the next six months is to agree on a replacement for the Lisbon Strategy - a blueprint that was supposed to make the EU the world's most competitive economy by 2010.
KEY 2010 DATES
18 Jan - Spain sets out priorities
11-19 Jan - European Commission nominees face parliament hearings, then MEPs vote 26 Jan
11 Feb - Special EU summit on economy
25 Mar - EU summit
18-19 May - EU-Latin America and Caribbean summit
25 May - EU-US summit
17 June - EU summit
Mr Van Rompuy will chair a special EU economic summit on 11 February.
Then on 25 March EU leaders will try to agree on the economic strategy for the next decade. It has already been outlined by the European Commission, which has called for a "smarter, greener social market".
Jobs and growth top Spain's agenda as the EU struggles to recover from the global economic crisis.
Spain's unemployment rate has doubled in the past two years. The rate hit 19.3% in October - the worst figure in the 16-nation eurozone and the second highest rate in the EU behind Latvia.
Another major challenge is to restore the EU's role as a world leader in the battle against climate change. The Copenhagen summit in early December was widely seen as a debacle for the EU, which wanted a much more ambitious final deal.
Treading on toes?
Spain has already signalled that it will let Mr Van Rompuy chair summits with third countries in Spain, with Spanish Prime Minister Jose Luis Rodriguez Zapatero at his side.
A key test of the new diplomatic protocol will be an EU-US summit in Madrid on 25 May. Who will shake President Barack Obama's hand first?
The world will be watching closely to see if the Lisbon Treaty improves or muddies the EU's international image.
The UK's Catherine Ashton will chair EU foreign ministers' meetings, as the new High Representative for Foreign Affairs. But first she must get the approval of Euro MPs at European Parliament hearings in January.
The 27 nominees for European Commission posts will each be questioned in turn by MEPs at the 11-19 January hearings, with a vote on the full Commission set for 26 January.
Resetting the compass
It is not yet clear to what extent Madrid will be able to focus the EU on Spanish foreign policy priorities, which include improving relations with Cuba and North Africa, forging closer trade ties with Latin America as a whole and pushing for the creation of a Palestinian state alongside Israel.
Spain's interest in the Americas and North Africa is already clear from the list of EU summits with third countries that will be held in Spain: Morocco (7-8 March), Canada (10 May), Mexico (15-16 May), Latin America and Caribbean (18-19 May), United States (25 May), and Egypt (5 June).
In July 2008 France - the then holder of the EU presidency - launched a 43-nation Union for the Mediterranean, which included North African countries, and that may get a boost from Spain's presidency. It made little progress in 2009.
But the EU has another major initiative with neighbouring countries - the Eastern Partnership, launched in May this year. It aims to build closer ties with six former Soviet republics: Armenia, Azerbaijan, Belarus, Georgia, Moldova and Ukraine.
Croatia is eager to conclude its accession talks with the EU in 2010, aiming for membership in 2011. Meanwhile, its Balkan neighbours are also in the queue to join at some future date, along with Iceland and Turkey.