Below you can find several possible ways to reach Groningen. Although Groningen has an airport (located in the village of Eelde nearby), it is highly unlikely that you will be able to get a flight there. The main option for flying is Amsterdam (Schiphol), and a possible low-cost alternative is Bremen (Germany, across the border), though in that case your options to get to Groningen from the aiport are a bit limited. Another option, perhaps most suitable for our German collaborators would be to take the train, or to drive.
This is our preferred way of traveling, as it is both the fastest, and least problems in case of delays in the flights. There are direct (or single transfer with a 3 minute wait) trains from Schiphol to Groningen, leaving at regular intervals, which take about 2 and a half hours. Upon arrival at Schiphol, while waiting for your luggage, buy a train ticket to Groningen (one way: € 27,30) from one of the yellow (with blue) machines. These machines are clearly marked in English with "Trein" (train) and "NS" (national railroad company), and should be easily locatable. Select destination Groningen main, second class, no discount ("zonder/geen korting") and the number of tickets (payable with cash, credit or debit card). After going through customs, make your way to Schiphol Plaza (impossible to miss, as can be seen from this map of Schiphol.
The departure times of trains are marked on electronic boards at Schiphol plaza, and more in detail near the tracks. The trains arrive and leave below Schiphol, so take the escalator down to track number 3 (track numbers are marked above the escalators). Trains to Groningen depart every 30 minutes, and a more detailed schedule can be found on the website of NS. Please pay attention to the small display on the side of the cars (near the top at front and rear of a car), indicating the final destination of that car. The direct train to Groningen (19 minutes past the hour) is at Schiphol still joined with the train to Leeuwarden (another city in the north of the Netherlands), and the train will be split at Zwolle (one and a half hours into the trip). Until that time one can still easily change from the Leeuwarden section to the Groningen section, but it is general convenient to be in the right section from the start (but if you have to run in Schiphol to catch the train, just get in, getting to the right part of the train is easy).
The other train (49 minutes past the hour) has a transfer at Amersfoort, which is normally a convenient transfer to the train across the platform (track 2 to track 1).Tickets are checked in the train (though not always), and the conductor is also the easiest person to ask about transfers, delays or if you are in the right car for Groningen.
Another option is to fly to Bremen, a German city about the same distance away as Amsterdam (150 km). Some low-cost carriers fly here (for example Ryanair). Getting from Bremen to Groningen is also very easy, since there are connections to the city both by bus and train. The bus connects Bremen Airport directly to Groningen Central Station and is usually not very crowded, but it only goes about once every 3 hours, so check carefully that your flight time matches a bus if you intend to go for this option. You can find the time table here. Roundtrips are € 30 and the travel time is about 3 hours. Tickets need to be bought beforehand on the linked website. Groningen central train station is the final stop, but there is one stop in Groningen just before (at the Oosterpoort). Taking a train can be a good alternative, but one has to take a tram from Bremen Airport to the station. From there, take a train to Leer and change trains for Groningen. The journey from Bremen to Groningen should take about 2 hours and 40 minutes.
Especially from Berlin or Münster this may be a viable option, though it should also be possible from Paris. The only international trains with Groningen as a destination come from Leer, but they tend to provide the fastest connection from northern Germany. If this is not an option, then one will generally have to transfer from high speed trains (ICE, Thalys) in Arnhem, Utrecht, Amsterdam or Rotterdam. Arnhem and Utrecht are both about 2 hours from Groningen, with Utrecht having a more convenient direct connection. Travelling to Groningen from Amsterdam Central Station or Rotterdam implies transferring once, usually in Amersfoort, but this tends to be a rather easy crossing of the platform. As mentioned above, trains from Amsterdam Schiphol can take you to Groningen directly.
Driving to Groningen should be easy, driving around in the center of Groningen is a disaster, and parking is limited. The center of Groningen has only one-way roads, and quite a few roads are forbidden for ordinary cars (only taxis/busses). Expect to park your car somewhere (€ 6 per day, € 34 per week) and travel inside Groningen on foot, by bus or by bike(!).
All of these options, except the last one should leave you in front of Groningen central station. The center of the city, most of the hotels and the meeting site are all within one kilometer walking, and can be fairly easily reached.
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