Header bidding is changing programmatic media by making every impression available to bidding in real-time and ensuring competition between ad exchanges and supply-side platforms. Previously with waterfalls, exchanges could not bid on impressions they did not see. Header bidding removes the waterfall and allows all partners to compete on every impression with actual bid values. This gives buyers access to 100% of a publisher's supply. Header bidding is facilitated by Prebid, an open-source project that helps publishers integrate header bidding partners and call bids asynchronously to reduce latency. Header bidding has increased competition and revenue for publishers when implemented correctly.