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Selective restriction of access to a place or other resource, allowing only authorized users

In physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, while access management describes the process. The act of accessing may mean consuming, entering, or using. Permission to access a resource is called authorization. Access control on digital platforms is also termed admission control. The protection of external databases is essential to preserve digital security. Access control is considered to be a significant aspect of privacy that should be further studied. Access control policy is part of an organization’s security policy. In order to verify the access control policy, organizations use an access control model.

Geographical access control may be enforced by personnel (e. g. border guard, bouncer, ticket checker), or with a device such as a turnstile. There may be fences to avoid circumventing this access control. An alternative of access control in the strict sense (physically controlling access itself) is a system of checking authorized presence, see e. g. Ticket controller (transportation). A variant is exit control, e. g. of a shop (checkout) or a country. The term access control refers to the practice of restricting entrance to a property, a building, or a room to authorized persons. Physical access control can be achieved by a human (a guard, bouncer, or receptionist), through mechanical means such as locks and keys, or through technological means such as access control systems like the mantrap. Within these environments, physical key management may also be employed as a means of further managing and monitoring access to mechanically keyed areas or access to certain small assets.

A credential is a physical/tangible object, a piece of knowledge, or a facet of a person's physical being that enables an individual access to a given physical facility or computer based information system. Typically, credentials can be something a person knows (such as a number or PIN), something they have (such as an access badge), something they are (such as a biometric feature), something they do (measurable behavioural patterns), or some combination of these items. This is known as multi factor authentication. The typical credential is an access card or key fob, and newer software can also turn users' smartphones into access devices. There are many card technologies including magnetic stripe, bar code, Wiegand, 125 kHz proximity, 26 bit card swipe, contact smart cards, and contactless smart cards. Also available are key fobs, which are more compact than ID cards, and attach to a key ring. Biometric technologies include fingerprint, facial recognition, iris recognition, retinal scan, voice, and hand geometry. The built in biometric technologies found on newer smartphones can also be used as credentials in conjunction with access software running on mobile devices. In addition to older more traditional card access technologies, newer technologies such as near field communication (NFC), Bluetooth low energy or Ultra wideband (UWB) can also communicate user credentials to readers for system or building access.

Access control decisions are made by comparing the credentials to an access control list. This look up can be done by a host or server, by an access control panel, or by a reader. The development of access control systems has observed a steady push of the look up out from a central host to the edge of the system, or the reader. The predominant topology circa 2009 is hub and spoke with a control panel as the hub, and the readers as the spokes. The look up and control functions are by the control panel. The spokes communicate through a serial connection; usually RS 485. Some manufactures are pushing the decision making to the edge by placing a controller at the door. The controllers are IP enabled, and connect to a host and database using standard networks

The most common security risk of intrusion through an access control system is by simply following a legitimate user through a door, and this is referred to as tailgating. Often the legitimate user will hold the door for the intruder. This risk can be minimized through security awareness training of the user population or more active means such as turnstiles. In very high security applications this risk is minimized by using a sally port, sometimes called a security vestibule or mantrap, where operator intervention is required presumably to assure valid identification. The second most common risk is from levering a door open. This is relatively difficult on properly secured doors with strikes or high holding force magnetic locks. Fully implemented access control systems include forced door monitoring alarms. These vary in effectiveness, usually failing from high false positive alarms, poor database configuration, or lack of active intrusion monitoring. Most newer access control systems incorporate some type of door prop alarm to inform system administrators of a door left open longer than a specified length of time. The third most common security risk is natural disasters. In order to mitigate risk from natural disasters, the structure of the building, down to the quality of the network and computer equipment vital. From an organizational perspective, the leadership will need to adopt and implement an All Hazards Plan, or Incident Response Plan. The highlights of any incident plan determined by the National Incident Management System must include Pre incident planning, during incident actions, disaster recovery, and after action review. Similar to levering is crashing through cheap partition walls. In shared tenant spaces, the divisional wall is a vulnerability. A vulnerability along the same lines is the breaking of sidelights. Spoofing locking hardware is fairly simple and more elegant than levering. A strong magnet can operate the solenoid controlling bolts in electric locking hardware. Motor locks, more prevalent in Europe than in the US, are also susceptible to this attack using a doughnut shaped magnet. It is also possible to manipulate the power to the lock either by removing or adding current, although most Access Control systems incorporate battery back up systems and the locks are almost always located on the secure side of the door. Access cards themselves have proven vulnerable to sophisticated attacks. Enterprising hackers have built portable readers that capture the card number from a user's proximity card. The hacker simply walks by the user, reads the card, and then presents the number to a reader securing the door. This is possible because card numbers are sent in the clear, no encryption being used. To counter this, dual authentication methods, such as a card plus a PIN should always be used. Many access control credentials unique serial numbers are programmed in sequential order during manufacturing. Known as a sequential attack, if an intruder has a credential once used in the system they can simply increment or decrement the serial number until they find a credential that is currently authorized in the system. Ordering credentials with random unique serial numbers is recommended to counter this threat. Finally, most electric locking hardware still has mechanical keys as a fail over. Mechanical key locks are vulnerable to bumping.

The need to know principle can be enforced with user access controls and authorization procedures and its objective is to ensure that only authorized individuals gain access to information or systems necessary to undertake their duties.

In telecommunication, the term access control is defined in U. S. Federal Standard 1037C with the following meanings:
A service feature or technique used to permit or deny use of the components of a communication system. A technique used to define or restrict the rights of individuals or application programs to obtain data from, or place data onto, a storage device. The definition or restriction of the rights of individuals or application programs to obtain data from, or place data into, a storage device. The process of limiting access to the resources of an AIS (Automated Information System) to authorized users, programs, processes, or other systems. That function performed by the resource controller that allocates system resources to satisfy user requests. This definition depends on several other technical terms from Federal Standard 1037C.

Special public member methods – accessors (aka getters) and mutator methods (often called setters) are used to control changes to class variables in order to prevent unauthorized access and data corruption.

In public policy, access control to restrict access to systems ("authorization") or to track or monitor behavior within systems ("accountability") is an implementation feature of using trusted systems for security or social control.