A Method of operating a dialogue system for serving complex user requests by appropriately controlling a network of devices (D1, . . . , Dn) and/or applications, wherein each device (D1, . . . , Dn) and/or application and its functionality are modeled by a finite state machine (FSM-1, FSM-2, FSM-3), which is using a plan language, wherein a reasoning component (1) is used in order to determine a plan (P) using said plan language for serving a received complex user request, wherein said reasoning component (1) is adapted for determining said plan (P) using said finite state machines (FSM-1, FSM-2, FSM-3), of said devices (D1, . . . , Dn) and/or applications, wherein a plan processing module (2) is used in order to perform a determined plan (P) for serving a received user request, and wherein said plan language is designed in order to enable the determination of conditional plans by said reasoning component (1), in particular by providing conditional planning commands.