1. A computer-implemented method of advertising for an advertiser via a user computer and multiple publisher systems, comprising: providing retargeting information to the advertiser for storage on the user computer, wherein the retargeting information comprises: multiple identifiers for multiple publisher systems authorized for retargeting; and an advertiser identifier; receiving from a visited publisher system user behavior information retrieved from the user computer, wherein the user behavior information includes the advertiser identifier; and transmitting an advertisement corresponding to the advertiser identifier to the visited publisher system for presentation via the user computer only if the visited publisher system is one of the authorized publisher systems.