The healing environment is a social psychological concept developed in the 1970s and 1980s for general hospital wards. In the Netherlands and the Scandinavian countries, these concepts were introduced in mental health care in the architectural design of Psychiatric Intensive Care Units (PICUs). This chapter first provides a historical overview of the concept of PICUs, then a historical overview of the healing environment. This is followed by a selection of studies looking into the association of ward design with mental health and some studies relating aggression to ward design. To wrap up, we provide an example of redesigning a PICU from the perspective of an architect and a ward nurse.