MST targets those youth offenders with serious externalising behaviour problems that are often co-morbid with mental health difficulties, substance use, and multiple family, school, and peer problems. For instance, many MST youth evidence any or all of the following difficulties: a family that is characterized by multiple needs and problems; extensive contact with mental health, child protection, special education, and/or youth justice systems; deeply involved with antisocial peers; has problems in school or does not attend; and abuses substances (alcohol, marijuana).

MST has proven effective in reducing antisocial behaviour among diverse populations of youth with high and complex needs. Research has shown that MST is effective with both male and female young people (and their families) of different ages, economic status, and cultural backgrounds.
Whilst MST has been shown to be an effective treatment for diverse populations of antisocial youth, it is not a “silver bullet” and is therefore not appropriate for all clinical populations. Accordingly, it is important to clarify which youth are appropriate for referral at the inception of any MST programme.

Inclusionary
Youth who meet any or all of the following criteria would be appropriate for MST:

• Target age range between 10 and 18 years (in some cases, younger ages have been included)
• Evidences externalizing behaviour problems or comorbid externalizing/internalizing behaviour problems
• Is at risk of out-of-home placement
• Has engaged in criminal behaviour and is involved in the youth justice system/correctional system
• Physical aggression in the home, at school or in the community
• Verbal aggression, verbal threats of harm to others
• School truancy
• School failure associated with behavioural problems
• Association with antisocial peers
• A mental health disorder in the context of the problems listed above
• Substance abuse in the context of problems listed above

Exclusionary Criteria (inappropriate referrals)

• Youth for whom a primary caregiver cannot be identified
• Youth whose sole presenting problem is sexual offending
• Youth in need of crisis stabilization because of active suicidal, homicidal, or psychotic behaviour. However, once stabilized (i.e., no longer considered to be in need of crisis intervention), these youth would be considered appropriate for MST
• Young people with Autism
• Young people with significant intellectual disability