The transcription factors of the Rel/NF-kappa B family play a central role in immune regulation. In addition, they may be involved in the pathogenesis of certain tumors, especially those of hematopoietic origin. In this review evidence is summarized indicating an association between dysregulated NF-kappa B-dependent gene expression and tumorigenesis. Furthermore, possible molecular mechanisms through which NF-kappa B might exert these effects, such as disturbance of cell cycle control or apoptosis, are outlined. Potential therapeutic approaches based on these findings are also described. Rel/NF-kappa B factors are presented here as an important example of the growing number of transcription factors possibly involved in oncogenesis.