Illegal migrants seeking to enter Europe use various long and perilous routes that are evolving as authorities attempt to stem the flow of new arrivals.
With migration at the centre of the European Union summit this week, here is an overview of how people are reaching the continent.
Most arrive by crossing the Mediterranean, according to the UN High Commissioner for Refugees (UNHCR), with 172,000 entering through Greece, Italy and Spain last year.
The number of arrivals is down sharply from the peak in 2015, when 1.02 million entered Europe via the Mediterranean, the agency says.
Since the start of this year there have been 44,370 arrivals; that is far below the 48,000 who entered Greece over just five days in October 2015.
Most of the crossings last year were from Libya or Tunisia into Italy, known as the "Central Mediterranean" route, says the EU's Frontex border agency.
It was used by 118,962 people, mostly Nigerians, Guineans and Ivorians, it says.
But arrivals via this route have plunged 75 percent since a controversial July 2017 deal between Rome and the Libyan coastguard.
Crossings have also dropped sharply from Turkey to Greece, the "Eastern Mediterranean" route. After close to 900,000 migrants in 2015, Frontex recorded only 42,000 last year, essentially Afghans, Iraqis and Syrians.
The decline came after a 2016 deal between the EU and Turkey in which Ankara agreed to take back illegal migrants landing on Greek islands in exchange for billions of euros in aid and other incentives.
More recently there has been a sharp increase in the use of a route between Morocco and Spain.
Nearly abandoned until 2000, this "Western Mediterranean" route saw 23,000 crossings last year, mostly of Algerians, Ivorians and Moroccans.
Since the start of 2018, 17,522 people have entered Europe via Spain, compared to 16,452 via Italy and 13,120 via Greece, the UNHCR says.
The main secondary route into the EU is through the western Balkans into Eastern Europe.
Around 12,000 people entered this way last year, according to Frontex, most of them Afghans, Iraqis and Pakistanis -- a sharp reduction from the 760,000 in 2015.
But this path joins up with one used by migrants who have already passed through Turkey and Greece, meaning the actual numbers of new arrivals don't always add up.
Routes into Europe are constantly evolving: according to the French embassy in Slovenia, a new one is developing from Greece through Albania, Croatia and Slovenia, with nearly 1,800 crossings between January and May this year.
An "Arctic route" was briefly in operation 2015 when about 100 migrants crossed a day from Russia into Norway on bicycle.
There are also several routes from African countries to departure points on the Mediterranean, some involving a dangerous crossing of the Sahara desert.
A Somali leaving from Mogadishu, for example, could travel via Addis Ababa, Khartoum, Cairo and then Tripoli, ahead of the sea crossing to Europe.
An Ivorian leaving from Abidjan might pass via Ouagadougou, Niamey and Agadez -- Niger's renowned "gateway to the Sahara" -- to reach Libya.
The UNHCR says it has registered 16,607 migrants dead or missing at sea since 2014.
To this should be added the toll for the perilous Sahara crossing, which the International Organization for Migration says is probably as high as that for the Mediterranean.
United for Intercultural Action, a Dutch group which records the identities of victims of the crossings, says at least 34,361 migrants have died trying to reach Europe since 1993.
"Thousands of others have never been found," it says.