"Milieu" is another way of describing an individual's or program's social "environment" or "culture". The social culture of an adolescent has a powerful influence on his or her behavior.

Milieu therapy is an approach that utilizes the social culture of the RTC to create positive changes in an adolescent. Utilizing the milieu as an intervention is one of the most powerful ways to effect change in residential treatment. This is achieved through therapeutic use of a resident's "community", which includes their peers, staff, community roles and responsibilities, groups, meetings and so forth. The positive influence of "peer pressure" can promote powerful and lasting change when combined with the skillful application of other therapeutic interventions.