Detaining foreigners is an administrative measure taken by the Aliens Office (Office des Etrangers.).
Rejected asylum seekers and migrants without a valid residence permit may be sent back to their country of origin or to a country through which they travelled before arriving in Belgium. With the objective of forced removal, they are arrested and detained in a detention centre, without prior sentencing.
Asylum seekers arriving at the border, in most cases by plane, are also systematically detained while awaiting their asylum decision which will grant them, or not, international protection and the Belgian residency permit that goes along with it.