This study aims to investigate the outcomes of children and young people referred to the UK Gender Identity Development Service and the factors associated with these outcomes. Specifically, the study aims to establish: (1) the proportion of children and young people who experience ongoing gender dysphoria and access medical treatment; (2) factors that are associated with ongoing gender dysphoria and treatment choice; and (3) how (A) medical treatment, (B) social transition and (C) co-occurring autism impact physical health, well-being, quality of life, peer and family relationships and cost to the National Health Service (NHS) and other public services. The study has recruited children, young people and parents/caregivers prior to being seen by the service and is committed to following them over time, regardless of whether families choose to remain in contact with clinical services or not. The qualitative sub-study of LOGIC, LOGIC-Q, aims to explore the experiences and well-being of a diverse sample of children and young people referred to the UK GIDS, and their parent/caregiver over a 2-year period. Specifically, the study aims to explore and examine (1) children and young people's experiences of gender identity, how this changes over time and links to health and well-being; (2) parental experiences of having a gender-questioning child, how this changes over time and support needed to promote their child's health and well-being; (3) children, young people and parental perspectives on outcomes that are important for evaluating health services and treatments for gender-diverse children; (4) children, young people and parental experiences of health services, therapies and support and how these can be enhanced.