Generally, not much. You need to file a flight plan, which is essentially just telling the ATC where you're going, and where you're coming from. The flight pla

Generally, not much. You need to file a flight plan, which is essentially just telling the ATC where you're going, and where you're coming from. The flight plan is also useful in case you get into trouble, since it contains the identifying information for your plane, the number of people on board, and for instance the safety equipment on board. So if you crash, the search-and-rescue people know what to expect. Depending on the system the place you're flying to uses, filing the plan might be filling a web form. At least in Finland you could also do that via a phone call or with radio. If there are customs and passport formalities between the countries you travel to, you will need to taxi your plane to a 'customs control' parking lot on the airport so that the customs people can check it. On some small airports this may require alerting them ahead of time so that they'll actually be there. You probably also have to have a radio and a transponder on the airplane. Between some countries, like the EU/Schengen area, flying between countries is pretty much the same as flying inside a country. So the requirements mostly depend on airspace restrictions in the area you fly. Between other countries things may vary. For example, back then I was told that it is quite impossible to fly from Finland to Russia since there was essentially no GA in Russia. (disclaimer: it has been nearly 10 years since I flew last time, so some of this may be incorrect or outdated)