Border crossing
National borders appear very different from each other. While closely associated countries such as the Netherlands and Belgium might be divided by a mere line in the pavement, borders with much political tension might be heavily fortified, such as the Korean DMZ.
In general, there are three kinds of border controls: Immigration, which checks people crossing the border, Customs, which checks goods crossing the border and Security which are for safety checks. Depending on the political situation, none, either one, or all may be enforced.
Immigration
Let man's petty nations tear themselves apart,
My land's only borders lie around my heart! – Chess
Here checks are made to see that you are allowed to enter a country. This will involve checking that your travel documents are correct, verify the reason for entering the country and the length of your stay. What further questions you will have to answer and how you are processed will depend on whether you are travelling just as a short term tourist, for work (short or long stay), study, emigration or seeking asylum. Checks may also be made on any criminal and political history (see travelling with a criminal history). At some borders evidence of travel to some countries (however circumstantial) may make you suspect to further questioning or being denied entry altogether.
Customs
Here you are checked to make sure you are not bringing into the country any illegal items (e.g. protected animals or products thereof or illegal drugs), items that you need to pay tax or duty on (e.g. alcohol, luxury goods) or items that need a permit (e.g. firearms and explosives). To prevent the transfer of disease, food stuffs and plant products are generally not allowed to be transported across borders; this can and often does include things like the apple in your bag or firewood in your truck. Be also aware that some customs sniffer machines are very sensitive, so do not eat a poppy coated bread roll (opiates are made from poppies and most drug tests give false positives after consumption of poppies) in Frankfurt airport before a flight to the Gulf countries.
On cross border ferries vehicles can be searched for illegal stowaways.
Some customs inspections are not related to international borders. For example, all travellers to California and Hawaii are subject to a (rather comprehensive) agricultural inspection, whereas most borders between EU countries don't have any customs inspections in the classical sense at all. You may well get picked out for spot checks, even well after crossing the border, if arriving from e.g. the Netherlands and fitting certain criteria.
If you have goods that need customs clearance and the border is less strictly checked (such as between Finland and Norway – or Åland!), you may have to visit a customs office quite a distance from your intended border crossing, possibly open only at specific hours (daytime, office hours or at ferry arrival).
Security
- See also: At the airport#Security check
Security checkpoints differ between forms of transport, and countries involved. For flights this obviously involves extensive checks on any potentially dangerous items, it is as strict as to include bottle of liquid, pocket tools and laser pointers. Again be aware of the sniffer machines, so make sure you have a shower and change your clothes after going hunting or being on the firing range.
On vehicle ferries there are often chemical and radioactive material scanners you need to drive though.
Road border security may be nothing more that just slowing down a little. Although the are no physical checks, there are usually cameras scanning vehicle number plates and sometimes facial recognition scans.
While most trains don't have any type of security check, Eurostar (between England and mainland Europe) and Chinese high speed trains do, so plan arriving half an hour or so early at the station.
In Eurocity cross-border trains inspections are usually done on the running train and you should have valid ID with you when boarding one of those trains.
Location of controls
Immigration and customs may not be directly on the border. For example, British passport and customs control for cross channel ferries are on the French side before boarding; and flights to USA from Canada, the US controls can be at the Canadian airport (the same goes for some airports in Ireland, UAE and the Caribbean too).
Customs controls may also occur where there is no national boundary, such as on entering and leaving a free port area. Customs controls may also occur within a country, for example Germany frequently checks vehicles (particularly foreign trucks) on their autobahns. International train connections between European countries are a favorite location for immigration officials to check visas, even if there are no formal immigration controls.
Security controls are obviously done before boarding the form of transport.
Free movement
Some (typically neighboring) countries have agreements that allow travel between them without having to pass immigration. The most famous one is the Schengen agreement that allows borderless travel between many (but not all) European Union and EEA countries.
The United Kingdom and Ireland, which are not members of Schengen, have a separate mutual free movement treaty.
The Central America-4 (CA-4) Border Control Agreement is a treaty between the Central American nations of El Salvador, Guatemala, Honduras and Nicaragua that allows the free movement of citizens and foreigners (with a valid visa for one of those countries).
The ASEAN group of countries in South East Asia are working on a visa free system, although it is unclear if and when that may come into effect.
Documents
Immigration control will for most countries require a passport and possibly some form of visa.
Some country's Customs control will require you to fill out a form stating such things as what goods you are planning to leave in the country or how much cash you are carrying.
By air
- See also: Arriving by plane
The airside of an international airport is a no man's land, where arriving passengers don't need to pass Immigration or Customs; however, Security might be enforced. Only when exiting the airport, one must go through Immigration and Customs.
Fees
Some countries, such as the USA, charge for immigration control costs. For air travellers this may be hidden in the price of the ticket, but if you cross by car or on foot, you will have to pay directly. In some (particularly low income) countries you will have to pay a fee even if arriving or leaving by air and this fee isn't included in your ticket price or comes on top of the fees that are. Such fees or taxes are charged for a variety of purposes, including to cover administrative costs, for environmental reasons, to discourage air travel, or to raise general funds. At some borders these fees can be paid by any variety of means (either currency or even a third one as well as credit and debit cards) whereas others insist on it being paid in national currency and small change. Wikivoyage aims to cover these things in the get in sections of country articles and border destinations, but if you see something that we don't or that is out of date, plunge forward.