The conceptual design of a three-dimensional, radiation oncology planning system is described. To assure that clinical needs were met, the working routines in two major Swedish radiation oncology departments were analysed in detail. Generic work flow was identified and mapped and compared to those in other institutions. The flow was partitioned into a number of nodes that together formed a basis for the design of the system handling logistics. The design criteria of this system emphasised accommodation of current clinical practice and traditional treatment modalities, and facilitated means to validate the computational techniques. The system should also allow for new procedures and was based on the analysis of current practice and a synthetic idea of how 3D treatment planning should be done. The final product supports the treatment planning work in its entirety. It is believed that the techniques followed are of interest to those engaged in computer systems of similar purposes and complexities.