Many people grow up with at least one sibling. These siblings are often ‘fellow travellers’ through adversity or significant life events; they can act as a source of support for some children while a source of conflict for others. For these reasons, siblings are a potentially powerful influence on development and this book is one of the first of its kind to provide an overview of cutting-edge psychological research on this important relationship. Why Siblings Matter is a cornerstone text on siblinghood. Integrating findings from a 10 year longitudinal study alongside wider research, it provides a lifespan perspective examining the impact of sibling relationships on children’s development and well-being. This text situates siblings in their historical, developmental and family context, considers the influence of siblings on children’s development and adjustment, and provides an introduction to new research on siblings in diverse contexts. The authors discuss sibling relationships in varied populations such as siblings with disabilities, siblings in different cultures and siblings in non-traditional families, while also considering the practical implications of research. Covering both classical studies and new results this book offers take-home messages for promoting positive sibling interactions. It will be invaluable reading for students and researchers in developmental psychology and family studies and professionals in education, health and social work.