Signal status display on the USS Excelsior; from top to bottom: "red alert", "yellow alert", "normal", "stand by", "auxiliary"

Aboard Starfleet vessels and outposts, a series of color-coded alert signals were issued, usually by the ship's computer or internal communications systems.

The Earth Starfleet began without an alert status system (signal) until one was suggested by Lieutenant Malcolm Reed aboard the prototype Enterprise, an early starship that had a larger crew and a greater need for reaction time during crisis situations.

While planning an escape from their hostage situation, Sub-Commander T'Pol mentioned that the sound of the transporter would alert the guards. (ENT: "The Andorian Incident")

Colored alerts

General alerts

A Galaxy-class alert indicator in normal, yellow alert, and red alert modes

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