Citizens of other European Union (EU) countries do not need work permits to work in Belgium. In addition, citizens of the EC community (the EU countries and Iceland, Norway, and Liechtenstein) are not required to apply for work permits. 

Citizens of all other countries must have a temporary visa and a work permit before settling in the country. The employer, not the individual, files the work permit application. The application must be filed in Belgium, and it requires the approval of the local labor department. Before a work permit is granted, the employer must prove that no Belgium or European Union citizen in the local market is able to do the job. The residency visa is granted after the work has been approved.

This is just a sample of what you'll find in the complete Belgium guide.