Recent clinical trials have shown us that it is possible to improve local control and survival by the concomitant use of radiotherapy and chemotherapy in a large variety of solid tumors, such as head and neck, lung, cervical, and anal cancer. The selection of drugs for this combined treatment, however, has been based on a rather empiric approach. Further research combining the clinical and laboratory expertise now offers the possibility of predicting and improving treatment efficacy for radiotherapy and systemic treatment, given alone or in combination. New methods, such as chromosome and gene expression profiling of individual tumors, are now becoming available with the comparative genomic hybridization assay and microchip DNA arrays. These assays will hopefully be of help in selecting patients for their optimal treatment regimen in the near future. Detailed knowledge of the mechanisms of action of these two treatment modalities will also lead to the development of new and more effective drugs, to be used concomitantly. It is the challenge of translational research to implement its opportunities into daily clinical practice. In analogy to the results obtained with concomitant radiotherapy and chemotherapy, this will lead to improved local control and survival rates in cancer patients.