National register number
The national register number is the unique identifier assigned to you when a Belgian commune enters you in the population or foreigners' register. It appears on the back of your identity card and is used by administrations, mutualities, banks, insurers and employers to identify you. Almost nothing administrative can be completed without it.
Why it comes before everything else
The number is created by the commune registration, which itself involves a residence check by the local police. That sequence is what makes the first weeks in Belgium administratively slow, and it cannot be short-circuited.
Once it exists, it unlocks the chain: affiliation to a mutuality, tax identification, itsme enrolment, and the completion of a bank file. Each of those parties uses the number as the key to your record.
It also encodes your date of birth in its structure, which is why it is treated as personal data and why organisations are only allowed to use it where the law permits. Do not publish it or send it casually.
Frequently asked questions
Where do I find my national register number?
On the back of your Belgian identity card or residence document, and on most official correspondence from public bodies. Your mutuality and your employer also hold it, since both use it to identify your file.
Can I get a bank account or health cover without it?
Health insurance affiliation is effectively impossible without it, and banks request it quickly for tax identification even if it is not demanded on the first day. Start the commune registration as soon as you arrive.