Liquid and solid olive mill effluents contain considerable quantities of organics, phenols and lipids. Comprehensive papers have been published on the treatment of olive mill effluents. Methane production and organic removal have been studied under varying environmental conditions. This paper reviews the recent reports on anaerobic reactors which have been published during the last 15 years. Olive mill effluents have high amounts of hardly biodegradable substances, with most of them being toxic to microorganisms. It has been proven that pretreatment with aerobic, advanced oxidation and heat methods are an efficient way of removing toxic materials and improving anaerobic treatment efficiency. The effects that organic loading, hydraulic retention time, and temperature have on suspended, biofilm, and granular reactors are discussed. Anaerobic treatment has been performed by feeding only olive mill effluents or co-digestion with other waste streams. Co-digestion enhances methane productivity by balancing nutrient and alkalinity levels. Furthermore, a comprehensive discussion of studies regarding pretreatment is carried out by comparing their performances. (C) 2015 Elsevier Ltd. All rights reserved.