The Massachusetts Youth Screening Instrument (MAYSI) was developed in the 1990s by Thomas Grisso and Richard Barnum at the University of Massachusetts Medical School with grants from the William T. Grant Foundation and the MacArthur Foundation. It was designed to be used within the juvenile justice system to identify youths experiencing thoughts, feelings, or behaviors that may need immediate attention regarding possible suicide risk or emergent mental health needs. Since being made available in 2000, the second version, MAYSI-2, has been adopted by over 1,500 juvenile justice facilities in 48 U.S. states to screen youths’ mental health needs when entering those facilities. This entry focuses on the development of, research behind, and use of the MAYSI-2 within the juvenile justice system.