Foreign workers who wish to work in Belgium as employees (i.e. working under an employment contract) must hold a work permit.
If you are an EU citizen coming from the following countries you may go and work in Belgium without a work permit: Bulgaria, Croatia, Cyprus, Denmark, Germany, Estonia, Finland, France, Greece, Hungary, Ireland, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Austria, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Czech Republic, United Kingdom, Sweden and Switzerland.
Still, in case you stay and work in Belgium longer than three months, you should sign up within three months after your arrival in the municipality of your residence.
For more information on living and working in Belgium visit Eures, the European employment services network.
If you are not an EU citizen contact the Belgian embassy in your country for more information.
For more information on working in Belgium (necessary formalities, work permit, free movement of workers, self-employment, recognition of qualifications, social security, taxes) visit belgium.be.