The Brotherhood of the Immanent Flesh was a secret organisation of people with psychic abilities. They were led by the Grandmaster. Their teachings were that the essence of flesh, of protoplasm, was upheld by a power, force or potential permanently pervading the universe; to embrace such an idea was to live life like art itself.
The Brotherhood acquired alien technology over the years. They used it to create a null envelope or psychic dead-zone. (PROSE: The Death of Art) They sent the reactivation signal that awoke the N-Form operating in the Quadrant (PROSE: So Vile a Sin) and at least one other, which destroyed the Quoth homeworld. (PROSE: The Death of Art)
According to one account, the Brotherhood was active in 1987. Dr. James Greco worked for the Brotherhood investigating Gabriel Tyler and his psychic powers, prompting them to steal UNIT file Quadrant27787/Doctor in December 1999. (PROSE: Damaged Goods) However, according to another account, Greco worked for the Torchwood Institute. (AUDIO: Damaged Goods)
The Brotherhood was still active in 2981. It took control of the Nexus on Mimas, planning to activate psychic abilities in everyone in the solar system. They believed the Doctor could help them control the Nexus. It was run by the new Grandmaster, a gestalt of twenty-eight telepaths that has existed for a considerable length of time; when any of the twenty-eight bodies died, the gestalt recruited a new one. The Brotherhood realised it needed to become proactive when the N-Form tried to wipe out psis on Earth in 1987. (PROSE: So Vile a Sin)