In order to legally live and start contracting in Belgium, there is a work permit and residence permit (depending on the length of your stay) which is required for certain nationalities.
Citizens from the European Union (EU), the European Economic Area (EEA – EU plus Iceland, Liechtenstein and Norway) and Switzerland can enjoy the freedom to work freely in Belgium without a Belgian work permit. A Belgian work permit is required for any non-EU/EEA/Swiss nationals going to work in Belgium.
There are three basic types of work permits (A, B, C including Blue Card) in Belgium. It depends on your situation as to which Belgian work permit is required. In general, there exists Belgian work permit A, Belgian work permit B, Belgian work permit C, Professional Card and European Blue Card.