Sibling relations, including the relations of both parents and children with their siblings, are of as great importance in assessing the normal or abnormal development of a child as in understanding family psychodynamics. Sibling interaction is one of the "universals" in a biography; it was even dealt with in mythology. Despite numerous studies, there is little conclusive evidence about the subjective and psychodynamic significance of birth order. Diagnostic and therapeutic assessments must be made with the particular case in mind, which also presupposes that investigators take into account resistance they themselves may have to the topic of siblings. The author discusses the points that can be of importance with regard to siblings.