The International Agency for Research on Cancer (IARC; French: Centre International de Recherche sur le Cancer, CIRC) is an intergovernmental agency forming part of the World Health Organization of the United Nations. Its role is to conduct and coordinate research into the causes of cancer. It also collects and publishes surveillance data regarding the occurrence of cancer worldwide.

Its IARC monographs programme identifies carcinogenic hazards and evaluates environmental causes of cancer in humans.

IARC has its own governing council, and in 1965 the first members were West Germany, France, Italy, the United Kingdom, and the United States of America. Today, IARC's membership has grown to 29 countries.

In late February 1963, after he experienced his spouse suffering and dying of cancer, journalist and peace activist Yves Poggioli sent a letter to Emmanuel d'Astier de la Vignerie relating his story, and urging support for the creation of an international center to fight against cancer, whose funding was to be directly debited from the national budgets allocated to nuclear weaponry. Touched by the letter, d'Astier assembled a group of French prominent figures, among which Pierre Auger, Francis Perrin, Jean Hyppolite, François Perroux, Pierre Massé, Louis Armand, François Bloch-Lainé [fr], Jean Rostand, François Mauriac, Antoine Lacassagne, Ambroise-Marie Carré and Le Corbusier, to reach for French president Charles de Gaulle in national newspaper Le Monde on the 8 November 1963. de Gaulle answered positively to the call and reached for the World Health Organization director Marcolino Gomes Candau on the 11 November. The project rapidly gained momentum, and IARC was created on 20 May 1965, by a resolution of the World Health Assembly, as the specialized cancer agency of the World Health Organization. The Agency's headquarters building was provided by its host in Lyon, France.

The first IARC Director was John Higginson (1966–1981), who was followed by Lorenzo Tomatis (1982–1993), Paul Kleihues (1994–2003), Peter Boyle (2004–2008), Christopher Wild (2009–2018) and Elisabete Weiderpass (2019–present).

In 1970, after IARC received numerous requests for lists of known and suspected human carcinogens, its advisory committee recommended that expert groups prepare a compendium on carcinogenic chemicals, which began publishing the IARC Monographs series with this aim in mind.

IARC identifies carcinogenic hazards based on qualitative assessment of animal and human evidence. The IARC Working Groups classify agents, mixtures and exposures into one of five categories. The categorization is a matter of scientific judgement that reflects the strength of evidence derived from studies in humans, experimental animals and other relevant data. The classification is based only on the strength of evidence for carcinogenicity, not on the relative increase of cancer risk due to exposure, or on the amount of agent exposure necessary to cause cancer.

There is sufficient evidence of carcinogenicity in humans. The determination is usually based on epidemiological studies on humans, but can also be based on sufficient evidence in experimental animals and strong evidence in exposed humans that the agent acts through a relevant mechanism of carcinogenicity. Examples of agents classified as Group 1 include tobacco smoke, alcoholic beverages, Chinese-style salted fish and consumption of processed meat.