When one begins to examine the existing literature dealing with siblings, one soon becomes aware that many separate domains of sibling research exist and that there is little connection between them; for example, sibling relationships in early childhood, genetic and environmental influences on individual differences between siblings, dysfunctional sibling relation­ ships, adult sibling helping relationships, sibling violence and abuse, and so on. The author's aim in writing this book was to attempt to bring together for the first time studies from diverse areas of sibling research into a single volume. The book is a summary and integration of the various domains of sibling studies, extending across the life span where studies exist to make this possible. Although many gaps in the sibling research literature within and between domains of study and over the life span still exist, it is hoped that this book will motivate others to help fill in the gaps by suggesting direc­ tions where further research is needed.