Nordic Passport Union

The Nordic Passport Union allows citizens of the Nordic countriesIceland, Denmark, Norway, Sweden, and Finlandto travel and reside in another Nordic country (and Svalbard) without any travel documentation (e.g. a passport or national identity card) or a residence permit. Since 25 March 2001, all five states have also been within the Schengen Area. The Faroe Islands are part of the Nordic Passport Union but not the Schengen Area, while Greenland and Svalbard are outside both.
However, Greenland has an open border with all Nordic countries, and allows Nordic citizens to enter, settle and work without requiring a passport or permits. Svalbard allows Nordic citizens to settle and work without permits, as a result of the Svalbard Treaty, however valid travel documentation (such as a passport, or a national identity card from a European Union or EFTA country) is required to enter Svalbard. Norwegian citizens may use other documents such as a Norwegian driving licence until 31 December 2021. Furthermore, as citizens of a Nordic country, those from Svalbard and Greenland are permitted to reside in any other Nordic country.
For Nordic citizens, no identity documentation is legally required to enter or reside within any Nordic country. However, identity documentation is still useful, as companies may require proof of identity for certain services, such as trains, airports, age check for alcohol purchase, or for services aimed at residents, like banking, picking up postal packages or dealing with authorities. Usually any valid proof of identity is accepted, in many cases local identity documentation like ID card from bank or other trusted private institute are accepted. An important exception is the "temporary" border controls which were introduced in 2015 and which as of 2020 still are in place.
When traveling to other countries than the Nordics, public officials in the foreign services of any of the Nordic countries are to assist citizens of another Nordic country if that country is not represented in the territory concerned, according to the Helsinki Treaty.
Establishment.
The Nordic Passport Union was established in three stages. In 1952, Denmark, Norway, Sweden, and Finland agreed to abolish passports for travel between them and to readmit citizens of other countries who had entered illegally into one of the four countries from another. On 1 July 1954, the agreement was extended to allow citizens to reside and work in any of the four countries without a residence or work permit. Iceland implemented the agreement on 1 January 1966. Passport checks for citizens of non-member countries at the borders between member countries were removed by a treaty between Denmark, Norway, Sweden, and Finland signed on 12 July 1957 and which came into force on 1 May 1958. The treaty was extended to Iceland on 24 September 1965 and to the Faroe Islands on 1 January 1966. Greenland and Svalbard remain outside the passport union.
Nordic countries and Schengen.
In December 1996 the two non-EU member states Norway and Iceland signed an association agreement with the signatories of the Schengen Agreement to become part of the Schengen Area. While this agreement never came into force, both countries did become part of the Schengen Area after concluding similar agreements with the EU. The Schengen Convention itself was not open for signature by non-EU member states.
From 25 March 2001, the Schengen "acquis" applied to the area of the Nordic Passport Union with the exception of the Faroe Islands. Border checkpoints have been removed within the Schengen Area, in addition to those that had been abolished previously within the Nordic Passport Union. In the case of the Faroe Islands, which are not part of the Schengen Area but remain part of the Nordic Area, the strict Schengen rules apply to travellers from the Faroe Islands into the Schengen Area, including the Nordic countries, but the Nordic rules allowing for an open border apply to those going the other way from the Nordic countries, but not necessarily from the other Schengen countries. However, citizens of the Nordic Passport Union countries enjoy extra rights within the Nordic area, not available under Schengen, such as less paperwork if moving to a different Nordic country, and fewer requirements for naturalization or citizenship. Within the Nordic area, any Nordic identity documentation (e.g. a driving licence) is valid as proof of identity for Nordic citizens because of the Nordic Passport Union, while a national identity card or a passport can be required in other Schengen countries. Most Scandinavian people do not own any Schengen approved national ID card, so they need a passport when visiting most Schengen countries outside the Nordic area, having in mind that most Schengen countries on the continent require possession of identity documentation by anyone, generally checked by hotels.
Before 2001, full customs checks were required for ferry travel when travelling from Denmark to other Nordic countries. There was no land border with Denmark before July 2000 when the Öresund Bridge was opened, which had relaxed customs check from the start. Nordic citizens did not need to be in possession of a passport. Other citizens needed a passport. As a part of the customs check, at least when arriving from Denmark and non-Nordic countries, every car driver was asked about where they had been. For land and ferry travel into Denmark and between Sweden, Norway and Finland, there were much more relaxed customs and passport checks, and often no checks at all. For air travel, there were full passport and customs checks, even though verbally claiming Nordic citizenship with a Scandinavian language, plus showing the ticket for the intra-Nordic flight, or any Nordic identity documentation, was usually enough to pass the passport check. It happened that Nordic citizens without a passport were not allowed to pass until a thorough identity investigation had been made. Passengers travelling by public transport, such as train or air, were usually not interviewed by the customs control. 
From March 2001, the Schengen rules have given more relaxed customs checks from Denmark, including no passport checks at airports, since intra-Schengen travellers are separated from the travellers from outside the Schengen area. Still laws give police and guards the right to demand an identity document at airports which is usually not enforced inside Nordic countries, at least not before 2015.
Agreements.
Abolition of passport for Nordic citizens.
The protocol concerning the abolition of passports for travel between Sweden, Denmark, Finland and Norway was signed on 14 July 1952, in Stockholm.
This protocol was implemented by the individual but concurrent decisions of the four governments. It abolished the need for citizens of the four countries to carry a passport when travelling in the three other countries provided that the trip was for such a short time that a residence permit was not necessary.
The agreement could be revoked in case of war, danger of war, or extraordinary international or national circumstances.
On 26 July 2014 the Norwegian police for the first time suspended this agreement due to a perceived terrorist threat.
Readmittance of aliens.
The agreement for the readmittance of aliens, was signed on 14 July 1952, in Stockholm.
The agreement provides that any alien, i.e. persons not citizens of the Nordic countries, having illegally entered one of the Nordic countries directly from another Nordic country should be readmitted by that country, unless that person has stayed at least a year in that country or has received either a residence or work permit.
Exemption of passport or residence permit.
The protocol concerning the exemption of nationals of Denmark, Finland, Norway and Sweden from the obligation to have a passport or residence permit while resident in a Nordic country other than their own, was signed on 22 May 1954 in Copenhagen.
The protocol was implemented by individual but concurrent decisions by the governments of each of the countries, and can be revoked in case of war, danger of war, or other extraordinary international or national circumstances.
The protocol provides that citizens of the Nordic countries can travel without passport or other travel documentation to any other Nordic country. Citizens of the Nordic countries can reside without residence permit in any other Nordic country.
Police authorities in the Nordic countries shall make all necessary information available to ascertain a person's identity and citizenship in a Nordic country.
Removal of passport checks at the internal Nordic borders.
The agreement between Denmark, Finland, Norway and Sweden to remove passport control at the internal Nordic borders, was signed on 12 July 1957 in Copenhagen and came into force on 1 May 1958.
This agreement removed all passport checks at the internal Nordic borders, and required the Nordic countries to uphold passport control at the external borders. Aliens with residence permits are allowed to stay up to three months in other Nordic countries, except for seeking employment or conducting business.
Aliens who are denied entry into one Nordic country should also be denied entry into the other Nordic countries. The Nordic countries are also obliged to readmit aliens that should have been denied entry at the first point of entry.
Temporary changes to border controls from 2015.
In November 2015, Sweden and Norway introduced temporary border controls, and requirements for all international airlines and ferries to check identity documents, due to the European migrant crisis. From 4 January 2016, Sweden required bus and train carriers to perform identity checks on the Danish side of the Denmark–Sweden border, while still retaining the border controls on the Swedish side. These controls were heavily criticised for violating the rules laid out in title III of the Schengen acquis prohibiting such carrier responsibility within the Schengen area, and were subsequently abolished by the Swedish government on 4 May 2017 to avoid potential formal sanctions. At the border, a traveller will need to be able to present a passport (and appropriate Schengen visa if required), an EU/EEA national ID card, or a Nordic driver's licence. However, identity cards recognised in the Nordic countries, like ID cards issued by banks or by the Swedish Tax Agency are not accepted.
The European Commission has been notified regularly of the ongoing temporary border controls at internal borders at every extension of the duration. As of November 2020 temporary border controls are still in place in Norway, Denmark, Finland and Sweden. However, the reason is now due to the COVID-19 pandemic.
This table refers to intra-Nordic passport controls, which violate the intention behind the Nordic Passport Union. They are legal according to the treaty since it is adopted to the Schengen agreement. Besides these intra-Nordic passport controls, Denmark has passport controls at its land border to Germany, and Sweden, Norway, and Denmark have passport controls on airports and seaports for arrivals from the continent.