Nearly 600,000 migrants have reached the EU by sea so far this year.
Turkey is hosting some two million migrants, most of them fleeing the war in neighbouring Syria.
Ahead of the summit, German Chancellor Angela Merkel stressed the need for a joint EU effort to tackle the crisis and said Turkey played a "key role".
"Most war refugees that come to Europe travel via Turkey. We won't be able to order and stem the refugee movement without working together with Turkey," she told the German parliament on Thursday morning.
BBC Europe correspondent Chris Morris says most EU leaders are convinced that efforts to contain the migration crisis will not succeed without closer co-operation with the Turkish government.
But, he adds, Ankara wants plenty in return - more financial aid to ease the strain of hosting refugees, as well as visa liberalisation and progress on Turkey's stalled application for EU membership.
The 28 EU leaders meeting in Brussels are hoping the Turkish government will sign up to a joint action plan that includes:
- Greater financial and procedural support for Turkey to deal with migrants
- Gaining permission from Turkey to help patrol its coastline
- Combating people smuggling
- Strengthening return operations
Turkey is expected to press for progress towards visa-free travel for its citizens to European countries within the so-called Schengen area.
The Turkish government has also called for the establishment of a safe zone for refugees in northern Syria - a move opposed by both the Syrian government and its ally, Russia.
European Commission Vice-President Frans Timmermans is in Turkey to discuss the crisis. "The EU needs Turkey and Turkey needs the EU," he said.
About two million people have fled to Turkey in more than four years of conflict in Syria. Every week thousands seek to enter the EU - typically via Greece - with many heading towards northern Europe.
Germany accepted the largest number of asylum claims in 2014 and expects to see as many as 800,000 this year. Sweden had the second-highest number of asylum seekers.
Conflicts in Libya, the Horn of Africa and Afghanistan are also fuelling the influx of migrants.
In a letter to EU leaders on Wednesday, European Council President Donald Tusk warned that the regional situation was "politically very complex".
He went on: "We must ask ourselves if the decisions we have taken so far, and the ones we are going to take on Thursday, are sufficient to contain a new migratory wave" - a wave, he warned, that could mean millions of new arrivals in the spring.