1. A method comprising identifying a location in a document that is to be used to present content, the location defining an environment in a document for presenting the content; defining a targeting criterion to be associated with the environment; and storing in the document a request code that is configured to request content for placement in the environment based on the targeting criterion; wherein defining at least one targeting criterion for the environment further comprises: selecting, by one or more processors, one or more sets of one or more advertisements from a plurality of advertisements; associating the selected sets of advertisements with the advertisement environment; and selecting an override criterion for overriding a selection of at least a subset of a selected set of advertisements in response to an advertisement request.