Mental illnesses are common worldwide and exceedingly complex, resulting from factors linked to biology, psychological conditions, personal choices and habits, and environmental influences. This chapter tackles mental and behavioral health (MBH) design and is organized into five main sections: defining MBH; an overview of MBH settings and patients; an evidence-based design MBH framework; design goals and strategies for MBH facilities; and exemplary projects that showcase these design goals and strategies. Based on the synthesis of the research for all MBH facilities, three physical environmental attributes were associated with the most positive behavioral outcomes: nature connection, proper light, and noise control. For inpatient MBH settings specifically, three physical environmental attributes were associated with the most frequently positive behavioral outcomes: flexible therapy spaces, nurse station observation, and private patient rooms. The physical environment plays a significant role in the health and well-being of MBH patients and the staff who care for them.