Physical security - controlling personnel access to facilities - is critical to achieving data center availability goals. As new technologies such as biometric identification and remote management of security data become more widely available, traditional card-and-guard security is being supplanted by security systems that can provide positive identification and tracking of human activity in and around the data center. Before investing in equipment, IT managers must carefully evaluate their specific security needs and determine the most appropriate and cost-effective security measures for their facility. This paper presents an overview of the principles of personnel identification and describes the basic elements and procedures used in security systems.