Digital political advertising – interactive, promotional content placed for a fee – comes in many forms, appears in a myriad of places, and can be targeted in many more ways than traditional television advertising. It includes display advertising (images, audio or video) and search advertising (based on key word search behavior); the goal may be to build supporter distribution lists, to fundraise for a candidate, party or specific political cause, to persuade, or to increase name recognition and distribute information. Paid political ads can appear as banners (across the top of the page), as page take-overs, on the side of a webpage or news feed, in mobile apps or in social media feeds where viewers can interact, like or share them, further disseminating paid content organically. Some online ads feature call to action buttons and some auto-play as pre-roll advertising, appearing before consumers can continue their online activity. Some can be skipped and others won’t allow further action until the ad finishes playing. Although describing the numerous forms digital advertising takes is cumbersome, it is arguably the easiest task in providing a systematic review of what is known about paid advertising online.