The cancer risk assessment of peroxisome proliferators (PPs) has been a matter of debate for decades. PPs are non-genotoxic carcinogens that cause liver tumours in rodents. There is considerable evidence that humans are refractory to the carcinogenic effect of PPs. Still, some toxicologists argue that these chemicals should be considered carcinogenic until the opposite has been proven. To solve this matter, much work has been dedicated to uncovering the mode of action of PPs. The proceedings are reviewed and it is discussed whether these provide an explanation for the observed interspecies differences and shed a new light on human health risk assessment. The efforts in the past years have significantly increased our understanding of the pathways involved, but have also raised new questions. The question why humans seem to be refractory is yet to be answered. From this point of view, the safety assessment of PPs to humans therefore remains an issue of discussion.