The book is about user interfaces to applications that have been designed for social and physical interaction. The interfaces are 'playful', that is, users feel challenged to engage in social and physical interaction because that will be fun. The topics that will be present in this book are interactive playgrounds, urban games using mobiles, sensor-equipped environments for playing, child-computer interaction, tangible game interfaces, interactive tabletop technology and applications, full-body interaction, exertion games, persuasion, engagement, evaluation, and user experience. Readers of the book will not only get a survey of state-of-the-art research in these areas, but the chapters in this book will also provide a vision of the future where playful interfaces will be ubiquitous, that is, present and integrated in home, office, recreational, sports and urban environments, emphasizing that in the future in these environments game elements will be integrated and welcomed.