Online advertising, also called online marketing or Internet advertising, is a form of marketing and advertising which uses the Internet to deliver promotional marketing messages to consumers. It includes email marketing, search engine marketing (SEM), social media marketing, many types of display advertising (including web banner advertising), and mobile advertising. Like other advertising media, online advertising frequently involves both a publisher, who integrates advertisements into its online content, and an advertiser, who provides the advertisements to be displayed on the publisher’s content. Other potential participants include advertising agencies who help generate and place the ad copy, an ad server which technologically delivers the ad and tracks statistics, and advertising affiliates who do independent promotional work for the advertiser. In 2011, Internet advertising revenues in the United States surpassed those of cable television and nearly exceeded those of broadcast television. In 2013, Internet advertising revenues in the United States totaled $42.8 billion, a 17% increase over the $36.57 billion in revenues in 2012. U.S. internet ad revenue hit a historic high of $20.1 billion for the first half of 2013, up 18% over the same period in 2012. Online advertising is widely used across virtually all industry sectors. Many common online advertising practices are controversial and increasingly subject to regulation. Online ad revenues may not adequately replace other publishers’ revenue streams. Declining ad revenue has led some publishers to hide their content behind paywalls.