This chapter presents an overview of the medical literature to outline what is known about gender dysphoria in childhood and adolescence. This chapter begins with a discussion of terminology commonly used in literature on gender diversity, as well as the preferred terms within this book. Second, literature on the medicalisation of gender diversity is reviewed to contextualise the relationship between gender and the medical establishment. Third, the diagnoses that may be given to trans children and adolescents that present to a medical professional for treatment are reviewed alongside critical commentary. Fourth, the research on gender variance in children and adolescents is reviewed broadly. Key themes identified in the literature include: prevalence; determinants of gender diversity; the natural history of gender dysphoria; and co-occurring conditions of trans youth. This area of research has advanced significantly in recent years. However, it is also in constant flux and answers to key questions are unsettled. This review of the medical literature provides context for treatment recommendations for children and adolescents discussed in Chapter 3.