Play is an integral part of preschool children's lives. Widespread concerns that children have less free, child-led play than in previous generations permeate the general public and academia, with parents criticised for: 1. limiting children's play opportunities by being overprotective; 2. not prioritising play with their children; and 3. choosing structured, enrichment activities rather than free play for their children. Preschool children's play is highly contextualised, yet how opportunities for play are created in the everyday context of family life has received limited attention. Mothers are often the primary caregivers and major decision makers regarding opportunities their children have for play. However, policy makers, educators, and health professionals have arguably neglected mothers' experiences underpinning these decisions.The foundations underlying these criticisms cannot be understood without asking:1. How do mothers describe, experience and participate in their preschool children's play? 2. What influences mothers' actions to create or limit opportunities for their preschool children's play? To answer these questions meaningfully, I took an ecological perspective using a constructivist grounded theory methodology. From 24 in-depth, semi-structured interviews with ten tertiary-educated, middle class mothers of preschool children (children aged three to five years who had not commenced formal schooling), I developed a grounded theory of How mothers shape the context of their preschool children's play. Using photo elicitation, eight mothers also shared photos of their children's play in the interviews.The theory explores the process by which the mothers constructed their actions in relation to their preschool children's play. These actions were based on a foundation of mothers' guiding motivations that were moderated through their perceptions of their child, their child's play and the context. By diagramming and grouping mothers' actions on two intersecting axes in terms of both the degree to which they facilitated or restricted play as 2. engaging meaningfully with their children through play, without being limited to direct play; and 3. fostering the developmental needs of their children, without over-engineering children's activities. Through the sharing actions of incorporating play in everyday activities and peripherally supervising their children's play in particular, mothers in the study created opportunities for their preschool children to play. These actions are typically associated with parents from developing societies, challenging the assumption that middle class parent-child play is characterised by direct play with children, a sharing action I labeled as co-playing.This constructivist theory provides novel insights into the process of how mothers shape the context of their preschool children's play and a new, data-driven understanding for families, policy makers, health professionals and educators seeking to facilitate preschool children's opportunities for play. Mothers are pro...