Digital advertising markets are growing and attracting increased scrutiny. This paper explores four market inefficiencies that remain poorly understood: ad effect measurement, frictions between and within advertising channel members, ad blocking and ad fraud. These topics are not unique to digital advertising, but each manifests in new ways in markets for digital ads. We identify relevant findings in the academic literature, recent developments in practice, and promising topics for future research. * Corresponding authors: zskatona@berkeley.edu , kcwilbur@ucsd.edu 1 Digital advertising markets have offered unprecedented innovations to marketers. Businesses can now advertise to finely targeted sets of individuals with customized commercial messages at specific locations and times in a variety of formats. Compared to traditional advertising, digital ads promise better targeting and relevance, personalized ad content, programmatic sales based on real-time auctions, and measurement of the co-occurrence of individual consumer ad exposures with a variety of online and offline response behaviors. These features have fundamentally altered marketers' spending: digital advertising revenues reached $108B in 2018, up 117% over 2014 (IAB 2019), with expectations to grow 19% and surpass cumulative traditional advertising revenues in 2019 (eMarketer 2019a). Digital advertising markets sell a wide variety of search and display advertising opportunities to marketers. Although digital advertising is 25 years old, market structures are still changing rapidly. For example, a census of marketing technology firms showed an increase from 150 in 2011 to 7,040 in 2019 (Brinker 2019). The IAB Tech Lab recently introduced a series of broad-based initiatives, including a new real-time bidding standard, and Google moved from second-price to first-price auctions for display ads. Publishers have introduced a variety of ad formats, with spending typically following consumer attention and media usage: after initial growth in desktop display and search, recent growth has been more concentrated in social networking, video, audio, and mobile ads.Yet there are indicators that unregulated markets for digital advertising have experienced problems. The E.U. has fined Google more than $9 billion in three antitrust cases and the U.S.Federal Trade Commission fined Facebook $5 billion after it broke a 2012 consent order (Case 19-cv-2184). Prominent politicians have criticized the industry and proposed structural reforms.