Abstract: A web site system includes an event history server system that persistently stores event data reflective of events that occur during browsing sessions of web site users, and makes such data available to other applications and services in real time. The server system may, for example, be used to record information about every mouse click of every recognized user, and may also be used to record other types of events such as impressions and mouse-over events. The event data of a particular user may be retrieved from the server system based on event type, event time of occurrence, and various other criteria. In one embodiment, the server system includes a cache layer that caches event data by session ID, and includes a persistent storage layer the persistently stores the event data by user ID. Also disclosed are various application features that may be implemented using the stored event data.