In this book, the Permanent Subcommittee on Investigations of the U.S. Senate Homeland Security and Governmental Affairs Committee examine issues of consumer privacy and security on the Internet and in the broader online economy. Central to this segment of the economy is the online advertising industry, which continues to grow in importance. In 2013, U.S. online advertising revenue for the first time surpassed that of broadcast television advertising as companies spent nearly $43 billion to reach consumers. The online advertising ecosystem is highly complex. Online advertisers do far more than merely disseminate text, graphic, or video advertisements. Underlying the work of online advertisers are sophisticated systems that are able to identify and target specific consumer groups with relevant advertising, as well as state-of-the art security practices to monitor the integrity of these ad delivery systems. The ability to target advertising is a key function of online ad delivery systems, and advertisers are willing to pay a premium of between 60 and 200 percent for these services. With the continuing boom in mobile devices, the importance, and complexity, of digital advertising is likely to continue increasing in years to come. The Subcommittee's investigation focused on the features and vulnerabilities in the online advertising industry that invite malware attacks. The Subcommittee also sought to highlight the potential hazards to private consumer information which result from consumer visits to even mainstream websites. The Subcommittee surveyed Internet participants and interviewed representatives from major ad networks, ad exchanges, data brokers, self-regulatory bodies, the Federal Trade Commission, consumer protection groups, and other participants in the online advertising industry to identify the vulnerabilities that have led to significant hazards to consumer safety and loss of consumer privacy online.