Merck & Co, Inc. is an American multinational pharmaceutical company headquartered in Rahway, New Jersey, and is named for Merck Group, founded in Germany in 1668, of whom it was once the American arm. The company does business as Merck Sharp & Dohme or MSD outside the United States and Canada. It is one of the largest pharmaceutical companies in the world, generally ranking in the global top five by revenue.

Merck & Co. was originally established as the American affiliate of Merck Group in 1891. Merck develops and produces medicines, vaccines, biologic therapies and animal health products. It has multiple blockbuster drugs or products each with 2020 revenues including cancer immunotherapy, anti-diabetic medication and vaccines against HPV and chickenpox.