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The purpose of this article is to examine how the process of self-other differentiation occurs in 3- to 5-year-old twins. To do so, a quantitative study (n = 264) was combined with a longitudinal qualitative study (n = 30 interviews) and a multi-source approach in which mothers, fathers, and twin children participated. For the quantitative section, a questionnaire was administered to the children ( Questionnaire des Relations Fraternelles [Fraternal Relations Questionnaire] (FRQ), Troupel-Cremel & Zaouche Gaudron, 2006) and another to the parents ( Pratiques Educatives et Relations Gémellaire [Educational Practices and Twin Relations] (PERG), Lamarque, Troupel-Cremel & Zaouche Gaudron, 2014). For the longitudinal qualitative study, five families were followed over a year and semi-structured interviews were conducted with both parents at three different times. The main findings indicate that self-other differentiation in twins can be viewed as either difficult to establish or as a continuum ranging from an absence of differentiation to fully established differentiation.