This book will cover theoretical and practical applications of socially engaging interactive systems either as virtual agents, or chatbots (intended as conversational voice interfaces), or robots physically occupying user's space. Theoretical determinants affecting user's intentions to use and accepts such systems will be considered such as user's social influence, accomplishments of important user's goals, hedonic motivations (the fun or pleasure of using a technology), price values (a trade-off between perceived benefits and monetary costs), and users' habits. In addition to these theoretical formulations, since the proposed interactive systems are increasingly more complex, showing humanoid and human appearance, the book is aiming to collect systematic investigations devoted to assessing the role of virtual agents, chatbots, and robotic features exploiting users' domain specific preferences in the context of healthcare and particularly in the context of applications devoted to easing and making independent the life of seniors living alone and vulnerable persons. Studies on virtual agent's acceptance, engagements in interaction and preferences in specific appearances such as human-like agents over machine- or animal-like ones will be reported, as well as the psychological effects that engage vulnerable people in emotionally believable interactions keeping off loneliness, sustaining health status, enhancing quality of life and simplifying access to future telecare services. Studies devoted to the testing how these interactive systems are accepted by a large population of users including young children, and adolescents, other than young, middle aged adults and seniors will be also considered.