Caller ID, known also as Caller Identification (CID), or more properly Calling Number Identification (CNID), is a telephone service, available on POTS (plain old telephone service) lines, that transmits a caller's number to the called party's telephone equipment during the ringing signal, or when the call is being set up but before the call is answered. Where available, caller ID can also provide a name associated with the calling telephone number, in some cases for a higher fee. The information made available to the called party may be made visible on a telephone's own display or on a separate attached device.

Caller ID may be used to track down or limit the impact of prank calls, telemarketers, and other intrusions. However, it can also impede communication by enabling users to become evasive. The concept behind caller ID is the value of informed consent; however, it also poses problems for personal privacy.

Another problem is that the possibility of caller ID spoofing makes it unreliable. In some countries, the terms caller display, calling line identification presentation (CLIP), call capture, or just calling line identity are used; call display is the predominant marketing name used in Canada (though customers often call it caller ID). The idea of CNID as a service for POTS subscribers originated from automatic number identification (ANI) as a part of toll free number service in the United States.

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