Operating in open and shared media, wireless communication is inherently less secure than wired communication. Even worse, mobile wireless devices usually have limited resources, such as bandwidth, storage space, processing capability, and energy, which makes security enforcement hard. Compared with infrastructure-based wireless networks, security management for wireless ad hoc networks is more challenging due to unreliable communication, intermittent connection, node mobility, and dynamic topology. A complete security solution should include three components of prevention, detection, and reaction, and provides security properties of authentication, confidentiality, non-repudiation, integrity, and availability. It should be adaptive in order to trade-off service performance and security performance under resource limitation. In this chapter, we will focus on the preventive mechanism for key management and broadcast authentication with resource constraints.