The concept of a Master Protocol for phase III studies was raised at the Steering Committee of the EORTC Radiotherapy Group, in order to make the work of the study coordinators easier, when writing protocols and to give them more homogeneity. The Master Protocol defines and clarifies in a logical order the different steps which must be taken when designing a randomized trial - from the rationale to the references. It pays particular attention to eligibility criteria, volumes of interest defined in agreement with ICRU Report 50 (gross tumor volume, clinical target volume, planning target volume and organs at risk), simulation procedure, treatment technique, normal tissue sparing, dose computation, equipment, dose specification (also in agreement with ICRU Report 50). Last but not least, the different procedures of quality assurance for protocols and patients are also defined (site visits, dummy run procedure, in vivo dosimetry, individual case review) to allow working plans to be made in advance. We are aware that this work is not exhaustive, but hope that the contents will be of help to those who are writing a protocol. © 1995.