This specification describes technologies relating to inferring interactions with advertisers. In general, one aspect of the subject matter described in this specification can be embodied in methods that include providing an online advertisement to a system, receiving data from the system indicating an interaction of the system with the online advertisement and data indicating a subsequent interaction of the system with an online resource of an advertiser associated with the online advertisement, and inferring that an advertisement conversion has occurred based on the received data. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.