A system for controlling a design process having a first design sub-process and a second design sub-process, outcomes of one of the first and second design sub-processes being linked to outcomes of the other of the design sub-processes by a relationship between one or more first design sub-process variables and one or more second design sub-process variables. The system comprises a user configurable interface between the first and second design sub-processes, the interface allowing a user to the system to control the design process by specifying which of the one or more variables are active variables which can have their domains modified by at least one process within the sub-process to which the variable belongs and which of the one or more variables are passive variables which have their domains determined within allowable values by the domains of the other variable(s) in the relationship.