The Slitheen may have been operating in Mongolia in 1952. The Slitheen were from Raxacoricofallapatorius orginally, and bribed their way into government. Their rule crashed the planet's economy and eventually the population rose up against them. Exposed, shamed, and arrested en masse in the Great Purge of Yon:556, the entire family was tried and found guilty, in perpetuity and given the death sentence. Many of the Slitheen fled justice, but the family continued to be hunted down by the Blathareen and Judoon.
Following this, in March 2006 they disguised themselves as various Humans holding leadership positions and attempted to seize control of the UK government in an effort to start a nuclear war. Their ultimate aim was to turn the Earth into a huge nuclear reactor pile and sell its remains off as starship fuel. This attempt was foiled by the Doctor and his allies. Later the same year, in September, Blon, one of their surviving members, managed to get herself elected as Mayor of Cardiff, and tried to abuse her position to attempt to blow up Earth by destroying a local nuclear reactor. The the Rift would amplify the explosion where it would not only destroy Earth but enable her to escape the planet. This effort was also foiled by the Doctor and his allies.Later, in September 2008, another branch of the family took over a building company called Coldfire Construction and attempted to achieve the destruction of the Earth by shutting down the Sun. This group of Slitheen were eventually defeated by Sarah Jane Smith and her friends. Though Sarah did not mean to kill them, several ended up dead. Korst Gogg Thek Lutiven-Day Slitheen, a young Slitheen survived, but not his father. In 2008, he teamed up with two other adult Slitheen to seek revenge and make the Moon crash into Earth. Torchwood Three knew of at least one of these events.
At some point in either early 2009, a young member of the family took the body of Ambassador Rahnius of the Galactic Alliance and attempted to steal K-9 from Sarah Jane. Later on, two more Slitheen would attempt to crush the Earth into a giant diamond; they were thwarted and captured by two Blathereen, who sentenced them to death.
One Slitheen was spotted in the Zaggit Zagoo bar, a Judoon was keeping an eye on the Slitheen.
By 2012, Henry van Statten possesed a Slitheen arm. A young member of the Slitheen family. A group of Slitheen returned to Earth in the late 21st century with hostile intentions but went home after a short time. After these attempts, their fortunes started to wane. They were put out of business by the Blathereen, since they were cheaper and more effective. In the year 2501, Ecktosca and Dram discovered the activities going on in Justicia and attempted to take over the operations. They were captured, but later escaped with the help of the Doctor. After the death of Don Arco, they stole the surviving warp-hole technology to rebuild their fortune.
In the 347th century, the family were still exiled from Raxacoricofallapatorius, although they expected a reappraisal in the coming 1000 years. When their time-travel package tour appeared to kill a tourist, they were taken to court and bankrupted.
The Slitheen placed great importance to hunting, which included the hunting of sentient prey. They sometimes displayed joy at triumph by disrobing. Blon said that the Slitheen used venom grubs to dispose of family members not willing to engage in criminal activities. A common tactic by the Slitheen to threaten their enemies in a confrontation is to grab one of them by the neck and threaten to kill them.
The Slitheen have developed a method of disguising themselves as humans by first obtaining the skin of a dead human -- referred to as a skin suit -- and compressing themselves into it. In order for this to work the human victim needed to be heavy set, and the compression had a side effect: a gas exchange occurs which causes the disguised Slitheen to frequently expel gas with a sound similar to human flatulation. In late 2008, the skin suit technology was improved to allow Slitheen to assume a greater variety of human body types, and the gas-exchange issue was eliminated. It wasn't always initially used, but would remain the standard until at least 2501.
- In 2012, Henry van Statten's private collection in the Vault contained a severed Raxacoricofallapatorian arm. One of their compression fields was also saved and studied by him. It had been found in Mongolia in 1952.
- Rose wonders if the newly regenerated Doctor was a Slitheen.
- Rose remarks that the Abzorbaloff looked "a bit Slitheen", and he claims to come from their twin planet, Clom.
- The Doctor asked Donna Noble if Lance Bennett was "A bit overweight with a zip round his forehead".
- The Journal of Impossible Things featured drawings of the Slitheen, with a caption saying "its always for the money".
- The Doctor referred to the Slitheen when Martha Jones recalled the earthquake in Cardiff.
- Jack Harkness congratulated Sarah Jane Smith on her defeats of the Slitheen.