In a system in which one or more client devices 10 access web servers 20 over a network 40, a classification server 30 is provided to assist in identifying the nature of the agent controlling the client devices 10. In particular, the classification server 30 monitors events that occur as a client device 10 accesses content on a web server 20, and uses a record of these events to classify the client device 10. This classification occurs when a classification time, generated in dependence on the events that have already taken place, is reached. The classification time represents the earliest time at which a classification can be made with a predetermined level of confidence.