Despite global advances in children's rights, young people are routinely disregarded, overpowered and excluded. This persistent discrimination - known as adultism - permeates family life, education, urban design, legal systems and political discourse. In this groundbreaking book, the authors provide a comprehensive introduction to adultism from an academic perspective while also emphasising its practical implications. Drawing on rich, real-world examples and research, they analyse it as a systemic form of discrimination, exploring how it evolved and is reproduced through language, institutions and everyday practices. Timely, accessible and urgent, this book offers a vision for resistance and transformation, outlining how adultism can be challenged - by both adults and young people - to co-create a more equitable future.