This chapter covers four areas. First, the author introduces, defines, and briefly describes how sexual and gender minority (SGM) polyamorous and consensually non-monogamous (poly/CNM) individuals, families, and communities have been researched in mental health studies. Second, the author reviews the available evidence-based research on poly/CNM SGMs and mental health, and mental health service provision. Third, the author reviews absences and erasures in research pertaining to the mental health of poly/CNM SGMs, particularly in relation to whether an intersectional lens has been applied in service provision such as in relation to age and to racial and ethnic minorities. In the final section, the author identifies and proposes future directions in the content and methodologies of research on poly/CNM SGM mental health, and recommendations for more effective mental health service provision.