WPP plc is the world's largest advertising agency group. Headquartered in London, WPP is a multinational communications holding company whose portfolio spans advertising, media investment, public relations, branding, research, commerce, and technology.
WPP's modern advertising business grew after founder Martin Sorrell acquired the Wire and Plastic Products company in 1985 and transformed it into a global marketing-services group. Rather than operating as one traditional agency, WPP owns and coordinates a large collection of specialist networks, including Ogilvy, VML, Grey, AKQA, and WPP Media.
The company is often compared with Omnicom and Publicis, which are also among the world's largest agency groups. The common mix-up is treating an individual agency, such as Ogilvy, as the group itself. WPP is the parent organization behind multiple major agencies and media businesses, so the question's wording refers to the group rather than one agency brand.
WPP is also frequently described as one of the advertising industry's Big Three, alongside Omnicom and Publicis.