Internet of Things (IoT) has been attracted more and more attentions in both academy and industry recently. It is envisioned as a key technology for extending traditional interconnection of human to interconnection of physical objects such as sensors, RFID tags, and mobile smart phones. A lot of novel applications come, e.g., smart grid, machine-to-machine (M2M), mobile cloud computing, EPCglobal networks, unattended wireless sensor networks, and mobile social networks. The security in those typical IoT applications is of upmost importance for their deployment in reality. In different applications, the security requirements or concentrations are various and challenges are distinct, e.g., privacy protection, trust management, authentication, key management, confidentiality, to name a few. This book analyzes the most important security requirements and proposes corresponding security solutions for those applications. In particular, all solutions are lightweight in terms of computation (communication) and robust in terms of resilience to compromising, so as to tackle corresponding challenges in different applications of IoT.