The vast majority of olive oil production (>98%) occurs in the Mediterranean region, utilizing it tremendous volume of water (10-30 million m(3)) in an area of the world in which water resources are limited. Treatment and reuse of olive mill wastewater (OMWW) presents significant challenges both due to the nature of olive oil production (seasonal and small scale) and due to the characteristics of the wastewater (high chemical oxygen demand (COD), high phenolic content, and dark color). A number of different microorganisms (Archaea, Bacteria and fungi) and processes (aerobic or anaerobic bioreactors, composting) have been tested to treat OMWW. Aerobic bacteria have been tested primarily as an approach for removal of phytotoxic compounds from OMWW, although some studies have also focused on reduction of COD. Fungi on the other hand, have proven effective at reducing COD and toxicity. Anaerobic consortia can effectively reduce COD, but are sensitive to phenolics in OMWW. Biological processes provide some of the most viable options for the treatment of OMWW. Effective application of these techniques, yielding significant reductions in COD, phenolics, and color, will allow safe and economical disposal of OMWW.