The transition of the advertising market from traditional media to the internet has induced a proliferation of marketing agencies specialized in bidding in the auctions that are used to sell ad space on the web. We analyze how collusive bidding can emerge from bid delegation to a common...

We present an empirical investigation of the role of marketing agencies in Google's online ad auctions. By combining data on advertisers' affiliation to marketing agencies with data on bidding in ad auctions, we analyze how changes in the concentration of clients in the same industry under the...