Metalworking fluids (MWFs) are widely used for cooling and lubricating during the machining process. The worldwide annual usage is estimated to exceed 2 x 10(9)l and the waste could be more than ten times the usage, as the MWFs have to be diluted prior to use. For UK industry the disposal cost is estimated to be up to pound16 million per year. Used MWFs cause high levels of contamination and rancid odours due to the presence of complex chemicals, biocides, etc., so that their treatment and final disposal must be handled carefully. Conventionally this has been done by combined physical and chemical methods but, with tightened legislation, these routes are no longer acceptable. Now, biological treatment is being increasingly adopted as it seems to offer an alternative with the potential for significant cost saving. However, there are significant difficulties in operating bioreactors, such as maintenance of the stability of the microbial communities present in activated sludge plants (ASP). In order to resolve these problems, four major areas need to be considered: (1) the composition of the spent MWF and its inherent biodegradability, (2) the recalcitrant compounds existing in waste MWFs and their impact on microbes, (3) the nature of the microbial consortia and means of optimising it, e.g, temperature and the practical design of the bioreactor and (4) the requirements for nutrient supplements and optimal control conditions. The potential importance of understanding the microbial community has been studied by the use of molecular biological techniques such as polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), fatty acid methyl ester (FAME) and fluorescent in situ hybridization (FISH). The application of attached biofilm bioreactors and thermophilic aerobic technology (TAT) has also been studied. This review describes recent advances in each of these areas.