A holding company is a firm that doesn’t actively engage in its own business but instead owns other companies’ assets, such as controlling amounts of stock. In the agency world, the term refers to a large international or multinational company that owns many advertising, public relations, media and other types of networks. The so-called “Big Six” holding companies in the brand marketing ecosystem are WPP, Omnicom, Publicis, Havas, IPG and Dentsu.