Siblings play an integral and essential part in our psychic development. Traditionally in psychoanalytic thinking, sibling relationships are regarded as secondary in developmental importance to the relationships with the parents. The authors in this book challenge this view and explore the impact of sibling relationships on internal psychic structures, family and social relationships. They suggest that siblings play a primary part in psychic development, even for an only child, and that infants are born with an expectation of siblings, an innate pre-conception similar to those relating to the breast and parental couple. Through infant observations and psychoanalytic treatment, the authors in this book examine sibling relationships from the most profoundly close, as in conjoined twins, through other twin and sibling relationships and deliberate on the wider context of social and tribal brotherhood and sisterhood.