A new user-interface paradigm for the specification of transfer functions is presented. The specification is usually a difficult task as mapping information for a number of different domains (data range, colour, opacity, etc.) has to be defined. In the presented approach, the definition of the mapping information can be realised independently for each property domain. A set of specification tools is provided for each domain, enabling users with different levels of experience or demanding time restrictions to choose an appropriate approach for their needs. Real-time feedback during the manipulation of parameters has been proven to be crucial to the specification. An interactive direct-volume-rendering display is realised by utilising dedicated hardware acceleration.