In recent years, Header Bidding (HB) has gained popularity among web\npublishers, challenging the status quo in the ad ecosystem. Contrary to the\ntraditional waterfall standard, HB aims to give back to publishers control of\ntheir ad inventory, increase transparency, fairness and competition among\nadvertisers, resulting in higher ad-slot prices. Although promising, little is\nknown about how this ad protocol works: What are HB's possible implementations,\nwho are the major players, and what is its network and UX overhead? To address\nthese questions, we design and implement HBDetector: a novel methodology to\ndetect HB auctions on a website at real time. By crawling 35,000 top Alexa\nwebsites, we collect and analyze a dataset of 800k auctions. We find that: (i)\n14.28% of top websites utilize HB. (ii) Publishers prefer to collaborate with a\nfew Demand Partners who also dominate the waterfall market. (iii) HB latency\ncan be significantly higher (up to 3x in median case) than waterfall.\n