This important book explores adolescent health and well-being through an ecological and developmental lens, drawing on international comparative studies and data sources to provide valuable contextual insights. Recognising that children and adolescents are not a homogenous group, this book highlights the importance of age distinctions in shaping health needs, risks, and protective behaviours. Adolescent development, the epidemiology and social determinants of adolescent health, health literacy and health promotion are foundational themes woven throughout the chapters. It explores how gender influences both health behaviors and outcomes, emphasising the need for a nuanced understanding of adolescent well-being. By presenting data comparisons across time and socio-demographic groups, this book sheds light on disparities related to gender, ethnicity (including Black and minority ethnic and Indigenous populations), age, geography, and socioeconomic status, offering valuable insights into the diverse factors that shape adolescent health. This textbook is intended for clinicians, researchers, policymakers, advocates, youth workers, adolescents and their parents and carers.