The evolving concepts of mobile computing, context-awareness, and ambient intelligence are increasingly influencing user's experience of services. Therefore, the goal of this paper is to provide an overview of recent developments and implementations of middleware-based pervasive systems, and to explore major challenges of implementing such systems. This paper also provides a comprehensive access to the literature of the emerging approaches and design strategies of middleware for providing users with personalized services taking into consideration their preferences and the overall operating context. Middleware systems were categorized according to their internal coordination model.