The second level of alert aboard Starfleet vessels, bringing the crew and defensive systems to a heightened state of readiness when danger is possible but not yet certain.
Yellow alert is the intermediate alert condition on Starfleet ships. It warns the crew that trouble may be coming and brings key systems to a higher state of readiness without sending the whole ship to battle stations, and it is often the step a captain takes on meeting an unknown vessel.
It is ordered throughout the original series, The Next Generation and their successors. Its limits are shown most sharply in Star Trek II: The Wrath of Khan. As the silent Reliant approaches, Kirk calls yellow alert but does not raise the shields, and Khan's first volley tears into the Enterprise's engine room.
If a threat is confirmed the ship moves up to red alert, and when the danger passes the captain orders the crew to stand down.