Non EU-nationals must have a work permit usually applied for by their potential employer, in order to be entitled to work in Brussels. Essentially there are three types of work permit in Belgium: A, B and C permit. Self-employed professionals from outside the EU must apply for a professional card (“Carte professionnelle-Beroepskaart). Without the appropriate permit you are not allowed to work or reside in Belgium.
Bear also in mind that family members of an EU citizen, trainees and au pairs have specific rules and obligations to follow.
Finally, it is important to know that employment is a matter that is regulated by the federal government but for which the regional authorities are competent. In the Brussels-Capital Region, the regional employment agency Actiris proposes a whole range of services to jobseekers.