Wireless Sensor Networks (WSN) are difficult to deploy. Limited energy, bandwidth, hardware and lack of security make WSN implementation difficult. Conventional security mechanisms cannot be applied because of the inherent processing and communicational constraints that exist in WSN. This book focuses on the design, implementation and analysis of an Optimized Security Protocol (OSP) that fulfills the requirements of confidentiality, authentication and integrity in WSN. OSP fulfils requirements of high level security without compromising resources. OSP architecture is based on the further optimization of computations in the Rabbit stream cipher and reduction in communication overhead to save sensor's life time. The OSP has been implemented using power of TinyOS coupled with NesC. In order to evaluate the system, several experiments have been carried out with respect to encryption/ decryption of various data blocks. Evaluation of OSP has been done by conducting a comprehensive efficiency analysis of the OSP architecture. Furthermore, the results of execution time and memory footprint for OSP have been compared to its cryptographic counterparts.