An important challenge facing behavioral health services is the lack of good quality, clinically relevant data at the individual level. The article describes a multinational research effort to develop a comprehensive, multidisciplinary mental health assessment system for use with adults in facilities providing acute, long-stay, forensic, and geriatric services. The Resident Assessment Instrument-Mental Health (RAI-MH) comprehensively assesses psychiatric, social, environmental, and medical issues at intake, emphasizing patient functioning. Data from the RAI-MH are intended to support care planning, quality improvement, outcome measurement, and case mix-based payment systems. The article provides the first set of evidence on the reliability and validity of the RAI-MH.