The book Wireless Sensor Networks focusses on various research challenges, solutions and variety of applications in Wireless Sensor Networks. It also gives an idea about various Hardware, Software and an operating system and three generations of a hardware platform designed to address the needs of Wireless Sensor Networks. TinyOS uses an event based execution model to provide support for finegrained concurrency and incorporates a highly efficient component model. TinyOS enables us to use a hardware architecture that has a single processor time shared between both application and protocol processing. A virtual partitioning of computational resources not only leads to efficient resource utilization but also allows for a rich interface between application and protocol processing. This rich interface, in turn, allows developers to exploit application specific communication protocols that significantly improve system performance.