Advertising has long been dominated by the "Big Six" holding companies: WPP, Omnicom, Publicis Group, Interpublic, Dentsu and Havas. These firms, the top six global advertising agencies by revenue, are responsible for a significant portion of all advertising content. Pure-play advertising firms maintained a stranglehold on the industry until 2014, when IBM iX became the first consultancy marketing arm to crack the international top 10 by revenue. Consulting firms have since pushed higher, placing four groups -- Accenture Interactive, PwC Digital Services, IBM iX and Deloitte Digital -- in the top ten, with Accenture Interactive outpacing Havas in revenue. This shift is born of a number of factors, most notably the inorganic growth of the consultancies and the increasingly challenging landscape faced by the agencies. Seeking to provide a more comprehensive suite of services to clients, consultancies have recently purchased many high-profile creative agencies. Traditional agencies, meanwhile, rely on a less-efficient business model ill-equipped for the digital age. By merging creative expertise with large-scale business solutions experience, consultancies formerly considered outsiders in advertising are gaining market share and delivering innovative, cost-saving solutions.