By expanding efforts of a larger ongoing research endeavor currently evaluating the efficacy of Reminiscing and Emotion Training (RET) via an RCT, this dissertation project directly extends prior work that has evaluated differences in mother-child play between maltreating and nonmaltreating families. Specifically, this dissertation evaluates differences in mother-child play between maltreating and nonmaltreating families during the preschool years at baseline and post assessment as well as whether specific baseline mother-child play attributes serve as moderators of RET intervention effects and children's functioning at post. 134 preschool-aged children and their mothers participated in baseline and 8-week post assessments with a repeated measures battery of interviews, observations, and assessments of maternal and child behavioral functioning. At baseline, maltreated preschoolers differed from their nonmaltreated peers only in offering significantly more initiations in play. At post, however, mothers and children in the RET treatment condition demonstrated significantly more positive engagement and responsiveness in play. Furthermore, baseline levels of mother and child negative engagement / responsiveness as well as of maternal attention directing in play moderated children's functioning (i.e., emotion regulation and positive self-concept consistency, respectively) at post. Together, these results provide critical information for informing and optimizing treatment and policy aimed at restoring adaptation for maltreated children and their families.