Olive mill wastewater (OMW) arises from the production of olive oil in olive mills. It is produced seasonally by a large number of small olive mills scattered throughout the olive oil-producing countries. OMW has a very high organic load, recalcitrant in nature and with a high amount of toxicity/phytotoxicity-associated compounds. Several physicochemical, biological and combined processes have been examined for the treatment of OMW, resulting in considerable organic load and toxicity abatement. Biological processes, aerobic and anaerobic, including anaerobic co-digestion with other effluents and composting, are predominant in the treatment of OMW. Advanced oxidation processes have attracted much attention owing to the strong oxidation potential of the agents used, which can result in a high degree of treatment. Recent research studies employing physical, chemical, biological and combined technologies are reviewed in the current work. (c) 2006 Society of Chemical Industry.