Caller identification (Caller ID) is a telephone service, available in analog and digital telephone systems, including voice over IP (VoIP), that transmits a caller's telephone number to the called party's telephone equipment when the call is being set up. The caller ID service may include the transmission of a name associated with the calling telephone number, in a service called Calling Name Presentation (CNAM). The service was first defined in 1993 in International Telecommunication Union – Telecommunication Standardization Sector (ITU-T) Recommendation Q.731.3.

The information received from the service is displayed on a telephone display screen, on a separately attached device, or on other displays, such as cable television sets when telephone and television service is provided by the same vendor. Value to society includes use by suicide-prevention hot lines and enabling businesses "like pizza restaurants and florists" to quickly have confidence in telephoned orders. The customer has control as to whether one's full name or merely first initial appears, a choice that to avoid a fee must be selected when the initial listing is generated.

Caller ID service, which is also known by similar terms such as CID, calling line identification (CLI, CLID), calling number delivery (CND), calling number identification (CNID), calling line identification presentation (CLIP), and call display, does not work with Centrex, a phone system widely used by corporations that allows outside callers to dial an extension without going through an operator.