Many laws specifically regulate the ways online ads are delivered. For example, online advertising delivered via email is more regulated than the same ad content delivered via banner ads. Among other restrictions, the U.S. CAN-SPAM Act of 2003 requires that any commercial email provide an opt-out mechanism.[110] Similarly, mobile advertising is governed by the Telephone Consumer Protection Act of 1991 (TCPA), which (among other restrictions) requires user opt-in before sending advertising via text messaging.
As advertisers collect data across multiple external websites about a user's online activity, they can create a detailed profile of the user's interests to deliver even more targeted advertising. This aggregation of data is called behavioral targeting.[27] Advertisers can also target their audience by using contextual to deliver display ads related to the content of the web page where the ads appear.[20]:118 Retargeting, behavioral targeting, and contextual advertising all are designed to increase an advertiser's return on investment, or ROI, over untargeted ads.[28]

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