In the science fiction of Iain M. Banks, Special Circumstances (SC) is an organisation that exists within the anarcho-socialist civilisation known as the Culture (which forms the basis of several of his novels and shorter works).
Special Circumstances is part of larger Culture organisation called Contact, which coordinates interactions with (and interventions in) other civilisations. SC exists to fulfil this role when circumstances exceed the moral capacity of Contact. Most of Banks' Culture novels concern themselves directly or indirectly with such circumstances.
Interventions by SC usually take the form of covert operations (military or otherwise) designed to strengthen or weaken factions within less advanced civilisations. These interventions are usually carried out by Culture citizens (The Player of Games), but are often "contracted out" to third party agents or mercenaries (Use of Weapons).
Given the extremely high standard of living within the Culture, these interventions serve as its "good works" to at least partially justify its hedonism. Typically, they have the explicit aim to improve the situation of less advanced civilisations, though sometimes interventions may also nip future challenges to the Culture in the bud (The Player of Games). While the Culture believes that it can statistically prove that most interventions achieve this end, operations are not always successful. Some (Look to Windward) are disastrous.
In times of war (Consider Phlebas), SC becomes especially powerful and, consequently, fashionable in the Culture. Its membership, usually very difficult to obtain, is one of the highest valued things for Culture citizens.
The morally ambiguous role of Special Circumstances is best explained by Diziet Sma in Use of Weapons :
"... in Special Circumstances we deal in the moral equivalent of black holes, where the normal laws - the rules of right and wrong that people imagine apply everywhere else in the universe - break down; beyond those metaphysical event-horizons, there exist ... special circumstances. [...] That's us. That's our territory; our domain."
The SC idea that an advanced civilisation should covertly help those less advanced is similar to the ideas and actions of Progressors in the Wanderers universe of Boris and Arkady Strugatsky.
The activities of SC are entirely at odds with the Prime Directive in the Star Trek universe. Starfleet explicitly prohibits activity of the kind sanctioned by the Culture.