In Belgium it is necessary to register at the town-hall of the municipality in which you are staying within eight days of arriving. Here you will be issued with a national identity card which has your national register number. Once you have received your national register number you are able to apply for an SIS card (Social Identity Card), which usually arrives in approximately 3 months, from one of the mutuelles. The mutuelles are the administers of social security. Linked to a political party, the mutuelles are called Christian, Liberal, Neutral and Socialist. You are free to choose any of the four and their benefits are very similar. They issue a card and when you visit a hospital, doctor or pharmacy, you pay upfront for the services and medications received, then you are reimbursed by the mutuelle usually around 85% of the cost.
This card has to be carried at all times and it is used as an identity for social security purposes. The card is valid for 10 years and contains information like name, date of birth, sex, social security number, SIS card number and validity. It is not usually required when opening a bank account or looking for a job. To find information on your municipality and where to register follow this link http://www.bruxelles.irisnet.be/fr/region/region_de_bruxelles-capitale/communes.shtml, which is available in English.