Systems and methods allowing publishers to define relationships between their own content items (e.g., World Wide Web pages) and content items created or maintained by other publishers. Publishers insert referral labels into their content items to indicate a referral relationship between or among publishers. In one embodiment, advertisements to be displayed while the user is viewing a content item are selected based at least in part on matching a referral label included in the target content item to a referral label in the advertisement. The referral label can also be used to manage referral-based revenue sharing among publishers.