HiFi Systems --- Network-centric query processing in the physical world Recent advancements in wireless sensors, RFID technology, and mobile devices have enabled the development of information systems that monitor and react to events in the real world. When deployed on a large (e.g., national) scale, these systems assume a high fan-in (or HiFi) architecture, in which large numbers of events measured at the edges of the network are continually refined, summarized, augmented, and aggregated as they flow towards the interior. HiFi systems present a wealth of new research problems reflecting the different concerns and priorities at each level of the system as well as the interactions among the levels. The solutions will require insights from recent efforts in data stream processing, sensor databases, event systems, data warehousing, and spatio-temporal data management. In this talk I will discuss some of the foundational work we have done in the Telegraph and TinyDB projects at Berkeley, and then speculate on how these and related efforts can serve as building blocks in the development of HiFi systems.