The World Wide Web portion of the Internet is largely supported by advertising. To deliver the most effective advertising, a system for dynamically creating customized advertisements is introduced. The behavior and any demographic information known about web viewers is used to select an advertising template that will be used to create an advertisement. The advertisement template comprises an incomplete advertisement with certain missing information along with identifiers for functions that may be used to complete the advertisement. In one embodiment, the functions may specify how the advertiser associated with the advertisement template may be contacted with the demographic information known about the user in order to fill in the missing portions of advertisement template. For example, the advertisement may concern flights to Hawaii and the advertiser may fill in the price of a flight to Hawaii based upon being provided with the user's location. The complete advertisement may then be displayed to the user.