Temporary extracorporeal liver assist devices based on liver cell cultures represent a promising approach for the treatment of liver failure as well as for perioperative care in liver surgery and transplantation. Primary hepatocytes from donor animals as well as human cell lines have been discussed as cell sources. A viable cell mass of 110-220 g (= 2.5-5 x 10(10) liver cells) has to be functionally active in bioreactors over a time period of several days. This cell number correlates to 10-20% of liver cells in an adult human liver. Neither in animal experiments, nor in clinical trials a complete liver replacement could yet be shown for cell-based bioreactors, however, beneficial effects have been demonstrated: partial detoxification and specific synthetic functions have been reported in animal experiments. Two authors have shown first preliminary clinical data on different cell-based liver assist devices with influences on blood parameters and the neurological status of treated patients. Nevertheless, clinical improvements also have been achieved by using non-biological support, like charcoal-hemoperfusion and modified hemodialysis. In order to assess the influence of liver support devices on better results in therapy of liver failure, randomized clinical trials have to be performed. Prior to this, artificial liver support systems have to demonstrate their biocompatibility and biosafety for clinical use as well as their efficacy and availability.