If you are an employer and you want to hire a foreign worker on Belgian territory, you have to obtain an employment permit in advance as a general principle. Your foreign worker, for their part, must hold a work permit B. This does not apply to nationals of the Member State of the European Economic Area or Swiss nationals: they do not need any employment permit nor any work permit.
Foreign workers can only work under some conditions in Flanders and Belgium. For an overview of these conditions, please check the website of the Departement voor Werk en Sociale Economie (Department of Work and Social Economy) or go to the provincial Dienst Economische Migratie (Department of Economic Migration) of the Departement voor Werk en Sociale Economie.
Thanks to the digital portal ‘Working in Belgium: Limosa’, administrative requirements are much less of a burden for foreign companies and organisations wishing to hire someone in Belgium or for people wishing to work as a self-employed either temporarily or partially in Belgium.
You have to file an application at your provincial Dienst Economische Migratie. This question is a federal matter managed by the Regions.
Companies based in Brussels should settle this question at the Directie Werkgelegenheidsbeleid en Meerwaardeneconomie (Employment Policy & Plural Economy Department) of the Brussels-Capital Region.
Nationals of the Member States of the European Economic Area and Swiss nationals do not need any employment permit. For more information about other exemptions and differences, you should ask your provincial Dienst Economische Migratie.