The prevalence of mental ill health in children and young people (CYP), both globally and in the United Kingdom, is high, and worldwide, as much as one in seven (13%) of 10- to 18-year-olds may experience a mental disorder. In the UK nations, there has been a marked increase in these conditions over recent years, and in England, more than 250,000 CYP are still awaiting treatment several months after their referral to specialist services. This introductory chapter will outline some of the background to mental illness in CYP, including adverse childhood experiences (ACEs) and risk and resilience factors, and will also identify future directions and policy initiatives for improving the mental health of CYP. These and other relevant mental health issues (e.g. the impact of social media use, transgender issues) will be explored in subsequent chapters of this book, which are also briefly described at the end of this chapter.