Wireless sensor networks (WSNs) are a promising technology that could induce a significant innovation in the field of structural monitoring. The main advantages of WSNs are fast deployment, little interference and self-organisation. However, since WSN are battery powered, the power management of the sensor nodes significantly influences the operation method and the overall data management process. Since data communication is the most energy-consuming task, a significant data reduction has to be attained in the sensor nodes to achieve system lifetimes that are useful for real life applications. This paper discusses several basic aspects of data processing and data management for long-term monitoring with WSNs. It presents a specific monitoring system and illustrates a long-term field test performed with this system on a bridge. The test results demonstrate that in-network data reduction is a very promising but challenging approach, since it has to be implemented with very limited computational and memory resources.