A cue light is a system of one or more electric light bulbs (usually traffic light colors) used to allow silent cues to be given to technicians and performers at various working locations during the running of a show. Typically, the Deputy Stage Manager sends signals to these cue lights at pre-arranged times. Sometimes these systems have an "Acknowledge" button that allows feedback on the status of the person at the cue point to the Cue Light Operator. Although headset systems have made cue lights less popular, they are still used in some cases where silence is necessary, or where a headset is not practical.

There are multiple protocols that are used to designate what each light means.

Green is usually used to signal a "go" cue.
An optional yellow light or a flashing red generally means "stand by."
A solid red light can indicate that the standby has been acknowledged, or that no cue is pending, depending upon the protocol used.
No light at all can represent that no cue is pending.

An alternate scheme with only one lamp uses "on" as a standby cue and "off" as the cue.

Many sound operator

Sound operator

The sound operator is the person responsible for the overall and total execution of all sound-related aspects of a theatrical performance...

s prefer cue lights to headsets so that they can hear the house sound accurately. Alternately, they will use a headset with a call light, rather than continually wearing it.