This study explores occurrence of playing within the field of relationship between a play therapist and a child-client. Based on Winnicott’s definition of psychotherapy, this study investigates it in the aspects of the areas of playing of the child and of the therapist, and of the overlap of the two areas. First, from the area of the child, playing functions as a main medium for emotional communication but can be interrupted at the moment of the child's sensing of a threat by reality. In particular, children with problems in their early attachment and/or with experiences of trauma often fails to create the field of playing but repeatedly enacts patterns of their earlier relationships within the field of therapeutic relationships, due to their exceedingly high or low levels of arousal. Second, from the area of the therapist, playing occurs when the therapist contains, through mentalization, the child's intense emotions and thoughts that the child cannot tolerate, and when he/she provides the frame of play mode that makes the child not feel overwhelmed by the threat, thus helping the child feel a sense of security. Finally, from the overlap of the two areas, playing occurs when a therapeutic relationship successfully forms as the therapist attunes sensitively to the child’s nonverbal cues, and as the therapist helps the child integrate fragmented parts of self that are enacted in the context of transference-countertransference.