Audience targeting is one of the foremost technologies to improve the performance of online advertising. With a view to the technical framework, the audience targeting tag is divided into three types: user tag, contextual tag, and advertiser-customized tag; their implementation plans are quite different. This chapter focuses on the former two tags, and the advertiser-customized tag is in essence the demand-defined tag when analyzing the demand-side platform technology. Some of these tags are obtained by simple calculation according to the parameter information in the ad request, such as geo-targeting, channel/URL targeting, and operating system targeting. After determining the technology to tag the contextual page, the page tagging system needs to quickly return its corresponding tag to a URL of Ad Server query during online ad serving. The granularity of audience targeting based on context can range from terms to pages.