Wireless sensor networks consist of small, mostly battery powered computers. Despite their simplicity, each sensor node is equipped with its own memory, CPU and radio transceiver. A typical application is to scatter many of them over a large area. Some sensor nodes can take measurements like temperature, air pressure and humidity. The latest models can also capture audio and images. But even the simplest capabilities like monitoring the temperature can be used e.g., to detect and fight forest fires at an early stage. The strength of this new paradigm comes from the mere number of nodes. Messages are forwarded over long distances from node to node. However, a sensor network does not only provide its own communication infrastructure. Within this book, it will also be shown how it can be used like a massively distributed database or as a compute cluster which filters and analyzes its data prior to transmission. A key-factor to the success of a sensor network is its longevity. Communication algorithms for medium access, routing but also for encryption and time synchronization have to be redesigned carefully with energy efficiency in mind.