The Madrid summit is designed to try to forge a consensus on how nations should work together to confront terrorism, while protecting democratic values and traditions. But even within Europe, ensuring co-operation has not been an easy task, as BBC security correspondent Gordon Corera reports.
An ivy-clad former monastery in the Netherlands houses Europol. Inside, teams of analysts pore over data provided by EU member states on criminal and terrorist activity, looking for signs of cross-border activity.
Its task is to try to get Europe's 25 police forces to talk to each other, and Deputy Director Jens Henrik Hojbjerg argues that the new threat requires new responses.
"Terrorists plan globally but act locally," he says. "And very often these people are not known to the local authorities, so you need co-operation.
"I think it's very clear that intelligence which is not shared and acted upon is basically useless."
After the Madrid bombings, Europol reactivated its counter-terrorist task force, but it still only has only about 30 analysts working on the subject, and there have been problems in getting countries to share sensitive intelligence.
But while Europol remains one tool of Europe's fight against terrorism, the Madrid bombings led to calls for a more co-ordinated approach.
Without a common approach, there is a fear that terrorists will simply move round the continent to whichever country they see as the easiest in which to operate or attack.
In the wake of the Madrid attacks, the EU brought in Gijs de Vries as the EU's first counter-terrorism co-ordinator.
He believes progress has been made. "We've seen an unprecedented increase in co-operation between intelligence and security services," he told the BBC.
"We've created in Brussels a centre for threat analysis of the terrorist threat, where we have analysts from intelligence and security services jointly sharing information at a European level, which is critical."
But there are still concerns that progress has not been fast enough.
Mr de Vries is urging EU member states to implement legislation "so we have common minimum standards in Europe and that allows us to have the scope to exchange info and work together.
"What countries do beyond that at national level is up to them, as long as it doesn't stand in the way of effective co-operation.
"What we haven't yet achieved is that all our member states implement some of our legislation.
"For example, the European arrest warrant which allows extradition of indicted criminals to proceed much more quickly than in the past - that has not yet been implemented by Italy."
But one major obstacle is that the way in which individual countries approach security policy often goes to the heart of sensitive national debates.
Europol says groups are actively pursuing weapons of mass destruction
How aggressive and intrusive should police and intelligence agencies be in monitoring places of worship or using stop and search powers?
What is the right balance between policies which are tough but which might in turn alienate communities and fuel extremism?
Across Europe, different states have pursued widely different strategies for dealing with the issue of terrorism.
'Threat is real'
This is due partly because of different perceptions of the threat, of the importance of civil liberties and of the right way in approaching minority communities.
In the Netherlands for instance, the killing of filmmaker Theo Van Gogh last November transformed the national debate.
As well as new social tensions, new tougher laws came in.
Last week, the government proposed new powers to track suspects through infiltration and detain them for even longer without charge.
As the Dutch case shows, local relationships and changing events make developing a common European approach to security a task fraught with difficulties.
In Brussels, Mr de Vries warns that while there have been successes so far, there are still significant dangers.
He says groups are actively pursuing the capability to use chemical, biological or radiological weapons.
"Over the last year, we've seen another increase in what our national governments have done to protect our people.
"We've seen another increase in what the EU has done to make our national authorities work across borders, but it is still not enough.
"The threat to Europe in terms of terrorist attacks remains real, and we'd do well to treat it seriously."