Creatures will randomly attack the vault in the form of incidents. This can also happen after failing a rush. Mole rats and radroaches multiply if there are no targets in the room, or if none enter the room within a short time after they spawn. Mole rats' and radroaches' strength depends on the level of the room in which they spawn. An average dweller level inside that room may also be a factor. Allowing the intruders to multiply will likely result in your vault's destruction. Radscorpions also appear in incidents. Unlike mole rats and radroaches, radscorpions don't multiply. Instead, they go to a random room if they are not killed fast enough in the previous room. They repeat the process until killed.
Creatures can also be encountered by vault dwellers while out exploring, but will not physically be seen in the game. Killing these creatures gives the exploring dweller experience. If the dweller is too weak for killing the creature and must escape, they still gain ten percent of the score.