After describing the general procedure for data encryption, which translates intelligible data into unintelligible form just prior to transmission and then reverses the process at a legitimate receiving terminal, the security features of various data transmission methods are described. These include fiber optics, which has the following inherent security features: (1) the light energy intercepted requires sophisticated processing to extract the desired information; (2) interruption of the signal by tapping is easy to detect; and (3) light energy does not leak electromagnetic radiation that can be captured and monitored. Optical laser links offer a high degree of security in that it is difficult to intercept the beam without detection. Also, shielding techniques can prevent electronic transmissions from being intercepted by unauthorized persons. The communications security checklist presented identifies some of the key issues to consider in establishing a security program aimed at protecting information and a communications system. The topics considered are technical surveillance methodology, information asset valuation, threat analysis, physical security, technical-surveillance-countermeasures surveys, electronic soundproofing, and encryption equipment evaluation.