On a short-stay holiday visa, it is not necessary to register once you arrive in Belgium. On other visas however, it is necessary to register, and registration is required within 8 days of arriving in the country and is done in the town hall of the municipality of where you are based. Depending on which municipality you are staying in, registration may be a short process as there are no lines, or may be a longer process if it is in one of the larger municipalities. At registration you will be issued with the type of identity document showing your status in the country and for how long it is valid.
These are usually required to be renewed every year.At registration you need to take your passport and all the material you needed to produce to get the visa in the first place. It is also recommended to take the passport and birth certificates of your spouse and children if you have any, as they also need to be registered as well. Please note that every time you change addresses in Belgium, you are required to register your new address in person in that municipality. For information on your municipality, visit the English version of the site: http://www.bruxelles.irisnet.be/.