Omnicom and Interpublic of New York, WPP of London and Publicis of Paris are considered the four biggest advertising groups in the world. Together they account for about 157,000 employees in well over 100 countries. Omnicom alone says it has 5,000 clients.
In 2007 the four reported total revenues of approximately $36 billion, the exact amount depending upon the conversion of euros and British pounds into dollars. That was nearly $2 billion more than Advertising Age reported they took in the previous year, when the magazine rated Omnicom first, WPP second, Interpublic third and Publicis fourth.
The four are more accurately identified as groups than as agencies because each includes several traditional agencies, including some of the oldest and most respected. All four were formed and continue to expand through mergers, acquisitions and affiliations. Each addition brings in new clients, offices and employees.