Viewability is one of the most important metrics publishers can optimize towards because the first step in driving brand engagement for advertisers is to ensure campaigns are seen by users. Advertisers use several key performance indicators (KPIs) to help measure the performance of their digital campaign spend. Compared to traditional advertising forms like radio and print, digital advertising offers unique opportunities for brands to understand how audiences engage with a brand via an advertising campaign. Advertisers measure performance through metrics that promote interactions with a brand such as the number of clicks on a banner or the amount of on-site purchases made after seeing an ad campaign. Since publishers often don’t have insights into the performance of advertisers’ campaigns, the best approach is to optimize for metrics that publishers can control which encourage buyer KPIs are met (such as viewability).