The present article examines the development of a diagnostic and therapeutic technique named The Sandtray at the Erica Foundation, a privately-run child counselling service in Stockholm. Originally it was called The World, developed by the British paediatrician and child psychiatrist Margaret Lowenfeld. In the 1930s it was imported to Sweden, where it gained a central position in child psychotherapy activities. The aim of the article is to discuss how The Sandtray was utilised at the Erica Foundation, both for diagnostic and therapeutic purposes. These two approaches are also discussed in relation to Foucauldian theories of the individualisation process. In the diagnostic practice the child was individually assessed and evaluated in relation to normative standards of scientific child developmental psychology, while in the therapeutic practice the individual child was through verbalisation techniques encouraged to reflect and demonstrate introspection as a subject. Two renderings of the individualisation of children are thus discerned in the utilisation of The Sandtray, one as a result of the disciplinary regime of the diagnostic practice, another through technologies of the self in the therapeutic practice. The latter also underwent a change with respect to how children were viewed. The emphasis on notions of children as different in comparison to adults was increasingly played down to the benefit of the notion of children as being the same. (Contains 76 footnotes.)