An interactive system, such as a webserver, is described of the type wherein responses to a user are associated with states of a state machine, the responses being generated as a result of a transition into a state associated with the response, the system comprising a state machine controller arranged to change states of the state machine according to the processing of user-initiated events. To provide for handling of concurrent events, the state machine controller is arranged, during processing of a current event, to determine whether the current event was initiated at an earlier time than another event and, if so, (i) restore the state machine to a condition consistent with that immediately prior to processing of the current event, and then (ii) carry out a state transition according the other event.