Based on socio-historic and ethnographic fieldwork, this thesis analysis the birth and evolutions of immigration detention in Belgium. In particular, the socio-historic approach sheds light on how the creation of the centres has to be replaced within the field of immigration policy-making in which different actors (both individual as well as institutional) vie for the symbolic power to define legitimate movement. From this perspective the creation of the centres is seen to have contributed to the autonomisation of this field. The immigration office (IO), which is the public administration in charge of the implementing migration policies in Belgium, benefited the most from the creation of the centres and as such became a hegemonic actor in the field of migration policy-making.The ethnographic approach then turns to the daily practices and organisation of the centres. In particular it looks at what the "humanisation" of detention changed in the daily organisation. I argue that the process of humanisation must be seen as the result of power struggles within the IO, which is considered as a field in which these actors struggles for the symbolic power to define legitimate detention policies. My data show how the process of humanisation has been one through which the security teams of the centres, who used to be hegemonic at their creation, have been dispossessed of their power to the benefit of management and social staff. The latter defined the visions of legitimate practices during my field work and as such occupied a dominating position in the organisational field.This shift however was only possible because social staff actually contributed to the elaboration and expansion of security in the centres. In particular I analyse how in their daily routines social staff actually functioned as risk managers because they had access to the biographical information of detainees, which they then put together and analysed to formulate guidelines on how to behave and interact with detainees on the basis of which management ...