Without Warning is a mission-based shooter (e.g., diffuse bomb, rescue hostage, etc.) that fuses weapon-based combat, puzzle-solving, and exploration through the corridors and main rooms of the compound, as well as some outdoor areas.

The environment is fairly interactive, therefore gamers can fire at lights to dim the room or target the undersides of huge ceiling-suspended barrels that will leak chemicals onto enemies. Players can also break open crates, which may contain goodies inside (such as a security keycard), or firing at exploding barrels may take out nearby baddies.