Middleware handles many important functionalities for ubiquitous computing. The authors distinguish various middleware technologies providing key elements for all applications' requirements such as discovery, adaptation/composition, context management, and management of ubiquitous applications. In this chapter, they propose a classification for some of the most employed ubiquitous middleware. The classification was established upon the challenges raised by ubiquitous computing - effective use of smart spaces, invisibility, and localized scalability - and upon how the various ubiquitous middleware respond to them in terms of interoperability, discoverability, location transparency, adaptability, context awareness, scalability, security, and autonomous management. This classification shows that if many middleware are mature enough and offer specific functionalities respecting the properties of ubiquity, a real lack is noticed in having an interoperable, autonomous and scalable middleware for the execution of ubiquitous applications. The development of the service-oriented paradigm, the semantics, the Web middleware, and the ambient intelligence shows the new trend the middleware research field is engaged in.