The Oxford Handbook of Sexual and Gender Minority Mental Health provides an overview of the current research on the mental health of sexual and gender minority (SGM) populations. It is aimed at researchers conducting studies on the mental health of SGM populations, clinicians and researchers interested in psychiatric disorders that affect SGM populations, clinicians using evidence-based practice in the treatment of SGM patients/clients, students in mental health programs (clinical psychology, psychiatry, clinical social work, and psychiatric nursing), and policy makers. The twenty-first century has seen improvements in sampling, use of longitudinal research, mixed methods research, statistical methods for research, and funding opportunities for research with SGM populations. Nevertheless, the purpose of this Handbook is to point out the gaps in the research as well as the advances, in order to motivate future researchers to expand knowledge about SGM mental health. As this volume goes to press, the current socio-political context in many nations includes both progress and backlash, with laws and policies including protections for SGM individuals in some countries, and laws and policies denying protections in others. All of these changes will impact SGM individuals, mental health researchers, and especially young people coming of age in this era.