As a retired person, a U.S. citizen, and a senior nomad, I’m officially a tourist. So how does that work, with regard to visas and such, in and around Europe?
Countries around the world like to have some control over who comes to visit them. Europe is a relatively small area and having border controls everywhere, especially for really small countries like Luxembourg, was cumbersome and expensive. In 1985, a number of countries in the fledgling European Union signed onto the Schengen Agreement, allowing open borders and free passage between countries. This has grown today to include 26 countries, including some non-EU members.

Citizens of one Schengen treaty country enjoy free access and unlimited stays in other Schengen countries. However, visitors from other countries operate on a Schengen tourist short-term visa, with time limits (and fees, for some). U.S. citizens are allowed to spend 90 days in any 180 day period in the Schengen area (with no fee) before they have to leave.
This can be complicated if you go in and out of the Schengen area frequently but, fortunately for me, I tend to arrive, stay, and leave. For example, I arrived in Spain (in the Schengen area) on September 4, 2019 and I’ll leave exactly 90 days later, on December 2, for the U.S. Then I’m forbidden to re-enter the Schengen area until another 91 days goes by, so December 2 + 91 days = March 1, 2020. This Schengen Calculator is a handy tool for figuring this out.
For variety, I could have spent six weeks in Germany and six in France (for a total of 90 days) and then exited the Schengen area.
Notice on the map above that there are six nearby non-Schengen countries: Ireland, U.K., Croatia, Romania, Bulgaria, and Cyprus. These countries also have their own 90/180 day limits but aren’t in the Schengen scheme. So my travel planning, if I want to stay in the European area, consists of hopping from the Schengen countries to the six non-Schengen countries and back every 90 days. An annual return to the U.S. for a month or so helps to balance everything out.

How much do these rules matter? Well, from what I read, it’s taken pretty seriously. You can be turned away at Customs and Border Control upon arrival if your entry-exit dates are off even by one day (as shown in your passport entry stamps and in their computers) and you can be deported. Having that “Deported” stamp in your passport is really undesirable, to say the least, and leaves a stink that doesn’t expire for three years. I recently made a mistake in calculating my Schengen dates, realized it, and had to cancel some reservations, which was expensive.
At one time, you could “reset” your Schengen days counter to zero by simply leaving the area for a day, but that was kiboshed by a change to the governing law in 2013.
The Internet is full of schemes for hacking the Schengen rules, most of which seem to depend on friendly or lazy customs officials and are too risky for my taste. There are also various other longer-term visas available (each country manages their own) for various lengths of time. Generally, these require student status, or family members who are foreign citizens, or owning foreign real estate, or depositing a lot of money in a foreign bank. Most of the rules are aimed at ensuring that you’re not going to a) work and take a job away from a native; and b) use the local healthcare system that you haven’t paid into. You could become a foreign or dual citizen, but I’m not going to get into that here. So for now, I must learn to be very careful with my date calculations.
This was the first, clear and simple explanation of the Shengen agreement that I have read. Thanks, Lee, for clearing that up for me.