Bayer AG is a German chemical and pharmaceutical company founded in Barmen, Germany in 1863. Today it is headquartered in Leverkusen, North Rhine-Westphalia, Germany. It is well-known for its original brand of aspirin. Bayer is currently the third largest pharmaceutical company in the world.

In order to separate operational and strategic management, Bayer AG was reorganized into a holding company in December 2003. The group's core businesses were transformed into limited companies, each controlled by Bayer AG. These companies are: Bayer CropScience AG; Bayer HealthCare AG; Bayer MaterialScience AG and Bayer Chemicals AG, and the three service limited companies Bayer Technology Services GmbH, Bayer Business Services GmbH and Bayer Industry Services GmbH & Co. OHG.

Following Bayer's successful reorganization, its chemicals activities (with the exception of H.C. Starck and Wolff Walsrode) were combined with certain components of the polymers segment to form the new company LANXESS. This change took place on July 1, 2004, with LANXESS to be listed on the stock exchange by the beginning of 2005. Bayer HealthCare's Diagnostics Division was acquired by Siemens Medical Solutions in January 2007.

Bayer AG shares are listed on the Frankfurt Stock Exchange, the London Stock Exchange and used to be listed on the New York Stock Exchange. On September 5, 2007, Bayer announced it intends to file for delisting of its American Depositary Shares (ADSs) from the New York Stock Exchange (NYSE). It is also planned to deregister with the U.S. Securities and Exchange Commission (SEC) and thereby terminate the respective reporting obligations.

Bayer is governed by a board of management, consisting of: Klaus Kühn, Wolfgang Plischke, Richard Pott, and Werner Wenning.

In 2004 Bayer HealthCare AG acquired the OTC Pharmaceutical Division of Roche Pharmaceuticals.

On March 13, 2006, Merck KGaA announced a €14.6bn bid for Schering AG. Merck's takeover bid was surpassed by Bayer's (successful) $19.5bn white-knight bid for Schering on March 23, 2006.

On March 11, 2008, Bayer HealthCare announced an agreement to acquire the portfolio and OTC division of privately owned Sagmel, Inc., a US-based company that markets over-the-counter medications in most of the Commonwealth of Independent States countries such as Russia, Ukraine, Kazakhstan, Belarus, and others. This deal is aimed at boosting Bayer HealthCare sales and market share in the above-mentioned region and expected to be finished by the end of 2008.