Reservations are required for many long-distance international trains, high-speed trains like TGV and Thalys and for night trains.
To check if a train requires a reservation, check an online timetable, it will indicate reservation requirements with a 'R' symbol or a sentence like 'Subject to compulsory reservation'. If in doubt, ask at a ticket window at the station before boarding to avoid paying a higher price on the train or even getting a fine.
In the section on Train Reservations you can find an overview of reservation fees and advice on how to make reservations.
Often you can avoid compulsory reservations by taking an alternative (slower) route. See the FAQ How to avoid train reservations.
