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Intermediaries are software entities deployed on Internet hosts of the wireline and wireless Web that intervene in the flow of information from clients to origin servers at the application level of the WWW. Intermediary systems fall under the general term of "middleware". Intermediaries represent a useful abstraction for the design and study of emerging software infrastructures for "next-generation" Web services. Their importance is increasing with the increasing demand for personalization, localization, and support for ubiquitous access over different physical media and protocols. We present an overview of a wide range of systems that can be described as intermediaries, classifying them in a number of broad categories according to their basic functionalities. Going beyond simple WWW proxies, we examine the requirements arising from the need to support personalization, mobility and ubiquity under high loads. We identify and refine a set of important properties and characteristics of intermediary systems. Based on these properties, we introduce a detailed taxonomy of characteristic systems and identify a number of key components of emerging intermediary infrastructures.