These monsters are scary as you look away from them you forget them.
The Silence are an unknown species as nobody knows where they actually come from. Over Series 5 alot of the monsters and villains have said about the Silence chasing them and making them run. The Silence are tall creatures who wear Black Suits and they only have 4 fingers. They are tall and their heads are like Husks.
The first time that the Silence was in 1962 when one appeared when Amy went to the ladies room and saw one there. Right infront of her a woman forgot she saw the Silent and payed the price. She was zapped by electricity by the Silent. Amy took a photo of it but forgot about it. The Doctor and the Companions ran off to an empty warehouse where the found some Spacesuits. River Song went underground and saw a group of Silence lurking in the tunnels but forgot about them when she looked away.
Then when the Doctor and his friends went back off into the Tardis he gave them a small device which recorded what they said. When Amy vanished the only thing that was left was the device. Canton Delaware the third shot a Silent to get answers which was when the Doctor found out their name. Amy was in a childrens room and saw a woman staring at her from the wall. She also noticed lots of Silence sleeping above her.
When the Doctor was captured by the US troopes his friends went into hiding. Whenever they saw a Silent they drew a line on their skin to show how many Silence they had seen. When the were all captured it really was just a trick. They travelled back to the warehouse where they went underground and followed a trail leading to a door. River and Rory entered along with the Doctor and Amy and saw a group of Silence with their own replica of the Tardis. When the Rocket went up into space a tricked wounded Silent said "You may as well kill us all on sight." Then the whole world saw them and for centuries remembered the Silence.The Doctor was pleased and ordered River Song to shoot them all and she did meanwhile he pretened to kill them with his Sonic Screwdriver.
In Series 6 the Silence returned with Madame Kovarian to make River Song kill the Doctor. The episode rewinded back when the Astronaut killed the Doctor. The Astronaut was infact River Song who couldn't kill him so time got muddled up.
When the Doctor was taken to Area 52 he noticed hundreds of Silence kept in glass cages underwater so that they couldn't use their electricity to break out and kill everybody. He noticed everybody was wearing eye patches and Madame Kovarian was captured. The Silence escaped and managed to escape and killed most of the Area 52 soldiers. Rory faced them bravely and killed 3 of them. When time was back to normal the Silence vanished. That was the last we saw of them.
The have appeared in:
Series 6 Part 1: The Impossible Astronaut and Day of the Moon
Series 6 Part 2: Closing Time and The Wedding of River Song