Refugees and migrants have been travelling to Europe in their hundred of thousands, fleeing conflict and persecution in Syria, Iraq, Afghanistan and elsewhere.
The extraordinary numbers of people arriving has placed huge strains on Europe and posed immense challenges to its governments and institutions.
362,775 Syrians applied for asylum in the EU for the first time in 2015
First time asylum applicants in the EU (Syrian citizens in 2015)
Arrivals to Europe via the Mediterranean
Data from the UN Refugee Agency
Dead or missing having attempted the journey
The huge spike, apparent at first in 2014 before accelerating to unprecedented levels in 2015, comes at a time of violence and unrest in the Middle East and in some African countries.
Between the third quarter of 2014 and the third quarter of 2015, Syrians made the most asylum applications by citizenship in the European Union. Since 2011, Syria has been devastated by a civil war between the brutal regime of President Bashar al-Assad and his now fractured opponents. The instability in Syria, and neighbouring Iraq, was seized upon by ISIS. Syrians, and Iraqis, fleeing to Europe are attempting to escape the horrors of their country - the horrors of Assad, of ISIS and of others.
Of the 997,125 first time asylum applications in the twelve month period between Q3 2014 and Q3 2015, 251,665 were by Syrians and 71,570 were by Iraqis.
The second most asylum applications by citizenship came from Afghanis (110,665) with Afghanistan another country beset by violence.
Search, rescue and patrol operations are in place in the Mediterranean but there continues to be incidents of shipwrecks and drownings on a regular basis with over 400 people dead or missing at sea so far this year. The Aegean Sea, which surrounds the Greek Islands, continues to be a hotspot for crossings with hundreds of people a day attempting the journey from the Greek coast.
For those fortunate to make it to Europe alive, the situation remains difficult. The European Commission has put in place a plan to relocate 160,000 refugees from Greece and Italy over two years from October 2015 but progress is painfully slow. As of January 2015, just 157 people of an initial 66,400 had been relocated from Greece and just 257 of an initial 39,600 from Italy.
A number of eastern European countries expressed opposition to the imposition of mandatory quotas on member states to take in some relocated refugees - Hungary's government said in February 2016 that it intended to hold a referendum on the matter.
In March 2016, the EU agreed a deal with Turkey aimed at easing the crisis.
The key aspects of the EU-Turkey deal:
From March 20th, migrants who arrive on Greek islands from Turkey who are not eligible for or do not apply for asylum will be returned to Turkey.
For each Syrian migrant returned to Turkey, the EU will relocate a Syrian from Turkey with priority given to those who have not previously tried to illegally enter the EU. The number of places available under the scheme is a maximum 72,000.
In return, EU will aim to lift visa requirements on Turkish travellers by the end of June and it will also speed up the dispersement of an initial €3 billion in aid. Turkey's accession process into the EU will be 're-energised'.
The humanitarian response
The UN's Refugee Agency expressed concern about the plan and said that the safeguards set out in the agreement were not yet in place.
When the provisional plan was published, Amnesty International accused leaders of having a 'persistent preoccupation' with sending people back to Turkey and said the plan was 'short-sighted' and 'inhumane'. When the final plan was agreed, Amnesty said it was flawed, illegal and immoral and warned that Turkey was not a safe country for migrants and refugees.
The huge numbers of people arriving has placed a great deal of strain on the EU's Schengen Area.
Within The Schengen Area people can travel across internal borders freely, without being subjected to checks on their passports. All EU countries are members of the Schengen Area, except for Bulgaria, Croatia, Cyprus, Ireland, Romania and the United Kingdom. Iceland, Norway, Switzerland and Liechtenstein, all non-EU countries, are also part of the Schengen Area.
Some EU states have reimposed border controls because of the large numbers of migrants and refugees crossing their territories. EU rules do allow states to temporarily impose border controls if there is a serious threat to public safety or internal security.
Denmark: Controls on borders with Germany - introduced in January 2016.
Norway: Controls on all internal borders, focus on ferries - introduced in January 2016.
Sweden: Controls on all borders, with a focus on certain harbours and the Öresund Bridge that connects Sweden with Denmark - introduced in January 2016.
Austria: Controls on all borders, with a focus on the frontier with Slovenia. Austria-Slovenia border can only be crossed at designated points - introduced in November 2015. Austria began restricting asylum applications to 80 people a day in February 2016 and said it would only allow 3,200 refugees a day to cross its territory.
Germany: Controls on all borders, with a focus on the Austrian frontier - introduced in November 2015.
France: Controls on land and internal air borders - introduced in November 2015 following the Paris terror attacks and ahead of the UN Climate Summit.
Belgium: Controls on the border with France as a precaution in case the French judicial system orders the deconstruction of the refugee camp in Calais, northern France - introduced in February 2016.
Reforming the EU's asylum system
In March 2016, the European Commission began a process of considering reforms to the so-called Dublin regulation, which requires refugees to claim asylum in the first EU country they arrive in - however the differing response of individual EU nations to the refugee crisis has thrown the system into chaos and confusion.
The Commission said it may propose scrapping the Dublin regulation altogether or amending it to include a 'corrective fairness mechanism'.