Physical security describes measures that prevent or deter attackers from accessing a facility, resource, or information stored on physical media. It can be as simple as a locked door or as elaborate as multiple layers of armed guardposts.

Elements and design

• obstacles, to frustrate trivial attackers and delay serious ones;
• alarms, security lighting, security guard patrols or closed-circuit television cameras, to make it likely that attacks will be noticed; and
• security response, to repel, catch or frustrate attackers when an attack is detected.

In a well designed system, these features must complement each other. There are at least four layers of physical security:

• Environmental design
• Mechanical and electronic access control
• Intrusion detection
• Video monitoring

The goal is to convince potential attackers that the likely costs of attack exceed the value of making the attack.

The initial layer of security for a campus, building, office, or physical space uses Crime Prevention Through Environmental Design to deter threats. Some of the most common examples are also the most basic - barbed wire, warning signs and fencing, concrete bollards, metal barriers, vehicle height-restrictors, site lighting and trenches.

The next layer is mechanical and includes gates, doors, and locks. Key control of the locks becomes a problem with large user populations and any user turnover. Keys quickly become unmanageable forcing the adoption of electronic access control. Electronic access control easily manages large user populations, controlling for user lifecycles times, dates, and individual access points. For example a user's access rights could allow access from 0700 to 1900 Monday through Friday and expires in 90 days.

An additional sub-layer of mechanical/electronic access control protection is reached by integrating a key management (physical key management) system to manage the possession and usage of mechanical keys to locks or property within a building or campus.

The third layer is intrusion detection systems or alarms. Intrusion detection monitors for attacks. It is less a preventative measure and more of a response measure, although some would argue that it is a deterrent. Intrusion detection has a high incidence of false alarms. In many jurisdictions, law enforcement will not respond to alarms from intrusion detection systems.