Playing is essential for the development of higher psychological functions. The purpose of this study is to demonstrate that playing constitutes an essential step in children's socio-cognitive development by describing playtime as the indicator of the gradual overcoming of self-centred thinking, with a view to acquiring new social adjustment strategies. The research activity included 64 children from nursery schools in Italy (average age 4.3) and was based on the direct and video-recorded observation of children's spontaneous play at school. Two coding systems were applied to analyse variations in ideation, exploration, socializing, and imitation activities. The preliminary results seem to confirm that playing provides a framework within which it is possible to articulate and express the child's progress in terms of both social development and cognitive skills. The mutual adjustment of the development of these two processes is also shown in the stepwise changing of the way in which conflicts are negotiated.