Palm oil mills generate about 3 t of effluent for every tonne of crude palm oil (CPO) produced or about 0.6 t of palm oil mill effluent (POME) for every tonne of fresh fruit bunch (FFB) processed. This effluent has to be treated to reduce its biological oxygen demand (BOD) before being discharged into the water course. Effluent pond digestion method is the most common treatment practice in almost all mills where the POME is digested by the microorganisms producing unpleasant odour, methane and other greenhouse gases. The objective of this study is to review the important metabolic pathways of microorganisms in order to predict the POME degradation under anaerobic and aerobic conditions.