ACNielsen is a global marketing research firm, with worldwide headquarters in New York City. Regional headquarters for North America are located in Schaumburg, IL. As of 2008, its the part of The Nielsen Company.

This company was founded in 1923 in Chicago, Illinois, by Arthur C. Nielsen, Sr., in order to give marketers reliable and objective information on the impact of marketing and sales programs. ACNielsen began expanding internationally in 1939, and now operates in more than 100 countries.

One of ACNielsen's best known creations is the Nielsen Ratings, which measure television, radio and newspaper audiences in their respective media markets. In 1950 they began attaching recording devices to a statistical sample of about 1200 consumer television sets in the U.S . These devices used photographic film in mail-in cartridges to record the channels viewed by the consumer and thus determine audience size. Later they developed electronic methods of data collection and transmission. In 1996, ACNielsen split off this part of its operations into a separate company called Nielsen Media Research (NMR), which operated as an independent company until it was acquired by Dutch conglomerate VNU in 1999.