WSNs are susceptible to a variety of attacks. These attacks vary in the way they are performed and executed; they include but not limited to node capture, physical tampering, denial of service, and message alteration. It is of paramount importance to protect gathered data by WSNs and defend the network against illegal access and malicious insertion of data that would alter the entire integrity of the system. The severe resource constraints in each sensor make it a challenge to secure the network. The need for new security ideas was the main inspiration and motivation of this research. While there has been a remarkable progress in many enabling technologies for sensor networking, the integrity of information received by the system has received less attention [50]. For instance, while many methods have been developed for self-organizing the network functions, less attention was paid to ensure high data integrity. While most the security mechanisms focus on confidentiality, we focus on integrity and freshness of the message. In order to achieve a secure system, security has to be integrated into every component. Sometimes security is viewed as a standalone component of a system's architecture, where a separate module provides security. This separation is a very flawed approach to network security. Components designed without security in mind can become a point of attack. The integrity model describes how data items in the system should be kept valid from one state of the system to the next. To define a security model, it requires specifying both the security requirements and the threat model. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]