This article reviews the fortunes of sibling theory in psychodynamic and attachment theories. Starting with points of conceptual overlap about the importance of the social environment on the child's personality development, cognitive function, and affect regulation, the article explores in turn the evolution of sibling theories in the psychoanalytic and attachment traditions. It highlights how despite profound conceptual and epistemological differences, researchers from both disciplines agree that siblings play an important forma- tive role in the development of the self and sense of well-being through the whole lifespan. These include the impact of the birth of a sibling; the crucial role of primary caregiver(s) in mediating and determining the nature of their family's sibling relationships; the consequences of neglect; and the particular resonance of sibling experiences in infancy and early childhood. The article explores how combining the conclusions from both disciplines might be helpful for clinical practice, including the relevance for their work of the practitioner's understanding of their own sibling legacy, and ways to think about sibling transferences and sibling narratives in the consulting room. It concludes with ideas for further research.