The milieu is a well discussed concept in the mental health nursing literature; so too is milieu therapy. Nevertheless, despite evidently clear understandings of the milieu's significance, and of its therapeutic potential, little published mental health nursing research focuses on these areas of practice. As a result, within the specialty, theoretical conceptualizations of the milieu remain unchallenged and theory development remains in its infancy. Findings from an inquiry into the practice of adolescent mental health nursing demonstrate the: (1) nature, (2) construction, (3) maintenance, (4) function, (5) therapeutic effects, and (6) outcomes of the milieu. Through these findings, present theoretical conceptualizations of the milieu are examined and expanded, and a beginning theory of the adolescent milieu, and of milieu therapy, is presented.