Theodore Millon (1969; 1991; 1994; 1996b; 2010) argues that predominant theories of personality tend to focus primarily on either the intrapsychic, interpersonal, environmental or biological aspects involved in personality development. Instruments, particularly objective, self-report questionnaires, are then developed in relation to these theories to assess personality. However, personality is more than just intrapsychic or interpersonal factors.

Millon attempts to combine the intrapsychic, cognitive and interpersonal spheres in his theory. He also acknowledges that an integrated approach needs to go beyond psychology if it is to be truly holistic (Millon, 1996b). In keeping with this argument, he borrows from evolutionary biology to develop his biopsychosocial evolutionary theory of personality, which underpins a number of instruments developed under the Millon umbrella.

The Millon family of instruments consists of pre-adolescent, adolescent, adult and medical patient inventories. The Millon Pre-Adolescent Clinical Inventory (M-PACI) is a comprehensive clinical tool that is designed to quickly and accurately identify psychological problems in children between the ages of 9 and 12. Unlike other instruments that focus on single clinical areas such as depression or anxiety, the M-PACI provides a synthesis of the child's emerging personality style and clinical syndrome, and can assist with early intervention (Millon, 2010). The Millon Adolescent Personality Inventory (MAPI) is a measure that assesses eight personality style dimensions, expressed concerns and behavioural correlates in normal adolescents aged 13 to 18 years (Millon, 2010). The Millon Adolescent Clinical Inventory (MACI) was developed specifically for use in regard to diagnostic assistance, treatment formulation and outcome measure in the clinical setting, and is used primarily for the evaluation of adolescents with difficulties. The MACI supplements the MAPI in providing a more holistic picture of the adolescent's personality type and clinical difficulties that he or she may be experiencing (Millon, 2010).

The Millon Inventories also include the Millon College Counselling Inventory (MCCI), Millon Behavioural Medicine Diagnostic (MBMD) and Personality Adjective Check List (PACL). The MCCI is an assessment tool that can help address students’ concerns and student-specific issues such as depression, stress, anxiety, substance abuse, suicidal ideation, and adjustment and relationship difficulties (Millon, 2010).