Chapter 1 introduces the topic of cancer in children, teenagers, and young adults. Cancer in children and young people is rare: less than 1% of total cancer incidence. There is a wide variety of tumour types, and these are often different from the common adult cancers. Leukaemia, brain tumours, and malignancies of embryonal origin are most common in younger children. Genetic predisposition is important. Environmental causes are less common than in adults. Treatments have improved significantly and, currently, about three out of four children and young people are cured of their cancer. Multimodality protocols including chemotherapy, surgery, and, more frequently now, biological treatment, in addition to selective use of more sophisticated radiotherapy techniques, is the norm. Increasing personalization of treatment based on risk stratification has allowed for improved cure rates with a reduction in treatment-related morbidity.