Nine more European Union countries have joined the Schengen area, which now allows passport-free travel across 24 countries.
BBC News website readers tell us what this means to them.
MICHAEL GORDON, BRITISH COUNCIL, SAUDI ARABIA
Schengen is "a lot of hassle" for Michael and his Burmese wife
This is not good news at all for us. My wife holds a Burmese passport, and it is extremely difficult for her to get into "fortress Schengen".
We seem to spend half our life trying to get her a visa to visit a Schengen country. Last summer it took us six weeks and a lot of hassle to get the visa for a two-week trip to France.
Two years ago we went to my brother's wedding in Slovakia. That was relatively straightforward then.
The Slovak embassy didn't charge us at all and she got the visa in minutes. Now that Slovakia is part of Schengen, we won't be going there again until my wife gets her UK passport.
We see Schengen as a fence designed to keep people out. Of course this will be very convenient for my brother's wife, who is Slovakian. She won't bother to get a UK passport at all now, I suppose.
BARBORA BEIJERS, ERMELO, THE NETHERLANDS
An Estonian carries balloons celebrating her country's entry
I live in the Netherlands, but I am originally from the Czech Republic. I remember very well the time in the past when we could not travel. The border was closed and we could only look through the barbed wire.
We could not imagine free borders so we have very good reasons to celebrate now.
I am happy!
ERLING NIELSEN, RISK CONSULTANT, DENMARK
I do not see the point of it all, to tell the truth.
We spend billions to guard our open societies against terrorism, but at the same time we open our borders.
This will facilitate organised crime, trafficking in drugs, humans and arms.
I do risk assessments on Eastern Europe and Russia and I know this will make it easier for criminals to become transnational in their operations. We should have more controls not fewer.
And you still need your passport or a form of identity anyway because of anti-terror legislation. They say it is for security reasons.
PETER VILMAN, KRANJSKA GORA, SLOVENIA
Peter took his passport along on shopping trips
I live in the tri-state area of Slovenia, Austria and Italy and I can tell you that this expansion will make life much easier for all of us living near the border.
People around here - including myself - do a lot of shopping in both Italy and especially Austria, in Villach, since there are loads of shops there selling everything from grocery to furniture and clothes. And it's closer for us to go there instead of going to the big shopping centres in, say, Ljubljana - an hour-and-a-half away.
Before this, you had to take either your official national identity card with you (which not many people have) or your passport, just to go and buy maybe a pair of jeans in Austria. But that was before the Schengen expansion.
Now I won't be obliged to take my passport with me just to go to the shops in another country or to run some errands.
I also look forward to the expansion as it will create a stronghold of security at our more important southern border with Croatia - the main link for illegal immigrants from the Balkans trying to get into the EU.
PAULO CASTRO GARRIDO, LISBON, PORTUGAL
I am from the Schengen area, Portugal, and I think Schengen is great.
Polish celebrations on the border with Germany
It gives the people the chance to move freely and to buy anything, wherever they want and how much they want without any restrictions.
While the internal borders are abolished, the external borders are reinforced and managed more efficiently. I met a UK immigration officer once and he told me that the Schengen area has the best database in the world on criminals.
The winners are Schengen citizens and the losers are those from outside the EU, who will have a lot more trouble trying to get in illegally.