This book has its background in a combined research and development project that led to the theorization of Play-Responsive Early Childhood Education and Care (PRECEC) (Pramling et al., 2019). On an overarching level, three main lessons emerged from this research. The first lesson was that there is often initial resistance among personnel in ECEC settings to play with children, a resistance related to different (implicit or explicit) understandings of key processes such as play, teaching, education, agency, and care. The second lesson learned was that, when looking at actual adult-child play, there are ample developmental opportunities and support that can benefit children in play and beyond (learning to play, learning in play, and learning from play). The third lesson was to acknowledge the value of empirical studies of play activities, in situ, to complement work based on ideology or views of play, children, and childhood. From these general observations, new questions emerged. These questions concern how other settings (e.g., formal/informal, in other cultural environments, in other adult-child relationships) relate to findings from ECEC context: What are general (recurring) patterns and what are important differences (and how may these be explained, theorized)? These questions propelled to the present book, which aims to provide empirically and theoretically grounded insights into what happens when adults play with children, and what are the potentials, consequences, challenges of, or resistances to doing so, with an international outlook. The book, as such, constitutes an important expansion beyond our own research and the state-of-the-art research field (for the latter, see e.g., Bruce et al., 2017; Bubikova-Moan et al., 2019; Smith & Roopnarine, 2019).