The main aim of this study is to investigate the development of play in children of 20 months old age, considering both the cognitive level expressed in play and the kind of interaction and affective support offered by the mother. We have evaluated the mother-child interaction during play by using a particular code structured in eight different categories; this code allows to point out the developmental sequence in child's play from simple functional activities to pretense behaviors with objects. The observation is conducted in a sample of 90 dyads of 20 months old, videorecorded at home, in their natural environment, during a semistructured free-play session of 10 minutes. The results point out the emergence of pretense at this age, even if there are many differences between children in term of frequency and duration of play activities at the different cognitive levels. Mothers tend to conform themeselves to child's play level, maintening the interaction by symbolic actions toward the baby; these play activities increase child's play.