In virtually all communication systems, the issues of authentication, confidentiality, and privacy are handled in the upper layers of the protocol stack using variations of private-key and public-key cryptosystems. These cryptosystems are usually based upon mathematical operations, such as the factorization into prime factors, believed hard to perform for an attacker with limited computational power; hence, we refer to the security provided by these systems as computational security. While computational security has a clear trackrecord of proven effectiveness, it may be difficult to implement in some emerging network architectures. For example, the advent of wireless networks has fostered the development of mobile adhoc networks comprised of many devices with heterogenous capabilities; the wide range of computing power available in the devices makes it difficult to deploy a public-key infrastructure. In contrast with the established practice of computational security, many results from information theory, signal processing, and cryptography suggest that there is much security to be gained by accounting for the imperfections of the physical layer when designing secure systems. For instance, while noise and fading are usually treated as impairments in wireless communications, information-theoretic results show that they can be harnessed to “hide” messages from a potential eavesdropper or authenticate devices, without requiring a shared secret key. Such results, if they can be implemented in a costefficient way without sacrificing much data rate, call for the design of security solutions at the physical layer itself to complement computational security mechanisms.