CQD (transmitted in Morse code as ) is one of the first distress signals adopted for radio use. On 7 January 1904 the Marconi International Marine Communication Company issued "Circular 57", which specified that, for the company's installations, beginning 1 February 1904 "the call to be given by ships in distress or in any way requiring assistance shall be 'C Q D' ".

A distress signal, also known as a distress call, is an internationally recognized means for obtaining help. Distress signals are communicated by transmitting radio signals, displaying a visually observable item or illumination, or making a sound audible from a distance.

A distress signal indicates that a person or group of people, ship, aircraft, or other vehicle is threatened by the serious or imminent danger and requires immediate assistance.: PCG D−3  Use of distress signals in other circumstances may be against local or international law. An urgency signal is available to request assistance in less critical situations.

For distress signalling to be the most effective, two parameters must be communicated:

Alert or notification of an emergency in progress

Position or location (or localization or pinpointing) of the party in distress.For example, a single aerial flare alerts observers to the existence of a vessel in distress somewhere in the general direction of the flare sighting on the horizon but extinguishes within one minute or less. A hand-held flare burns for three minutes and can be used to localize or pinpoint more precisely the exact location or position of the party in trouble. An EPIRB both notifies or alerts authorities and at the same time provides position indication information.