In politics, opposition research (also called oppo research) is the practice of collecting information on a political opponent or other adversary that can be used to discredit or otherwise weaken them. The information can include biographical, legal, criminal, medical, educational, or financial history or activities, as well as prior media coverage, or the voting record of a politician. Opposition research can also entail using trackers to follow an individual and record their activities or political speeches.

The research is usually conducted in the time period between announcement of intent to run and the actual election; however political parties maintain long-term databases that can cover several decades. The practice is both a tactical maneuver and a cost-saving measure. The term is frequently used to refer not just to the collection of information but also how it is utilized, as a component of negative campaigning.

In the 1st century BC, Cicero is said to have gathered information that was damaging to opponents and used it in attacks against them. He accused one political opponent, Catiline, of murdering one wife to make room for another. He attacked Mark Antony in speeches known as the Philippicae, eventually prompting Antony to chop off his head and right hand and display them at the Roman Forum.

Opposition research also has its origins in military planning, as evident in such ancient texts as The Art of War, published in the 5th century BC by Sun Tzu. This manual for warriors describes the necessity for understanding an opponent's weaknesses, for using spies, and for striking in moments of weakness.

In 18th-century England and Ireland, opposition research took the form of scandal-mongering pamphlet wars between the Whig and Tory parties. Writers such as Daniel Defoe, Jonathan Swift, and Henry Fielding participated, often writing under assumed names. This tradition of robust attack was replicated later in the American colonies, when writers such as Thomas Paine and Benjamin Franklin conducted opposition research and published their results.

The first appearance of the phrase "opposition research" in The New York Times occurred on December 17, 1971, in an article that describes the infiltration of the Edmund Muskie presidential campaign by a female Republican volunteer: "...an article appeared in a Washington newspaper describing the 'opposition research' program at Republican headquarters..."

Opposition research became systematized in the 1970s, when Ken Khachigian, a speechwriter in the Nixon Administration, suggested that the GOP keep files on individuals as insurance against future races, rather than "scramble" in an ad hoc fashion, race by race.

Opposition research differs immensely depending on the size and funding of a campaign, the ethics of the candidate, and the era in which it is conducted. Information gathering can be classified into three main categories: open-source research enabled by the Freedom of Information Act, covert operations or "tradecraft, " and maintenance of human systems of informants. Increasingly, data-mining of electronic records is used. Information is then stored for future use, and disseminated in a variety of ways. A local election sometimes has a staff member dedicated to reading through all of the opponents' public statements and their voting records; others initiate whisper campaigns that employ techniques of disinformation or "black ops" to deliberately mislead the public by advancing a pre-determined "narrative" that will present the opponent in a negative light.