There is a growing consensus that mental health is not merely the absence of mental illness, but it also includes the presence of positive feelings (emotional well-being) and positive functioning in individual life (psychological well-being) and community life (social well-being). We examined the structure, reliability, convergent validity, and discriminant validity of the Mental Health Continuum-Short Form (MHC-SF), a new self-report questionnaire for positive mental health assessment. We expected that the MHC-SF is reliable and valid, and that mental health and mental illness are 2 related but distinct continua. This article draws on data of the LISS panel of CentERdata, a representative panel for Longitudinal Internet Studies for the Social Sciences (N = 1,662). Results revealed high internal and moderate test-retest reliability. Confirmatory factor analysis (CFA) confirmed the 3-factor structure in emotional, psychological, and social well-being. These subscales correlated well with corresponding aspects of well-being and functioning, showing convergent validity. CFA supported the hypothesis of 2 separate yet related factors for mental health and mental illness, showing discriminant validity. Although related to mental illness, positive mental health is a distinct indicator of mental well-being that is reliably assessed with the MHC-SF. © 2010 Wiley Periodicals, Inc. J Clin Psychol 00:1–12, 2010.