Mental health professionals (N = 83) employed at 2 inpatient settings participated in a study on the nature and justification of assessment and treatment decision making. Clinical psychologists, psychiatrists, a psychiatric nursing service staff group (comprising registered nurses, licensed practical nurses, and nursing assistants), social workers, nonpsychiatric physicians, physician assistants, mental health workers/psychiatric technicians who had at least a high school diploma, rehabilitation specialists, and psychiatric administrators completed a treatment decision questionnaire constructed by the authors. It addressed several specific content areas relating to the types of assessment procedures, treatment goals, and treatment methods mental health professionals usually use, as well as the usual reason(s) for such procedures. Results showed that inpatient mental health professionals mostly relied on past success as well as logistical–practical factors in the determination and justification of assessment and treatment methods. Analyses of differences among inpatient institutions and mental health professions are also presented. The conclusion was that, regardless of the specific assessment and treatment methods relied on, mental health professionals did use systematic decision procedures in choosing such methods.