The pulp- and paper-industry generates large volumes of highly heterogenous wastewaters containing compounds from wood or other raw material, process chemicals and compounds formed during processing. The wastewaters from mechanical pulping and secondary fiber pulping as well as the condensates from chemical and semi-chemical pulping are typically non-toxic to methanogenic degradation and contain easily degradable organic compounds. Consequently, anaerobic digestion is an attractive treatment alternative for these effluents. In addition, both primary and secondary sludges from pulp- and paper-industry wastewater treatment-plants are amenable to anaerobic digestion. In contrast, the bleaching effluents from chemical pulping, the debarking effluents as well as the CTMP effluents are likely to be inhibitory to methanogenic degradation; also their biodegrability is relatively low. Dilution with other wastewater streams or detoxification by various pretreatments have been used to facilitate anaerobic treatment of these inhibitory wastewaters. The potential of the anaerobic systems for reductive dechlorination and sulfur recovery is unique and of great interest. In almost all pulp- and paper-industry full-scale applications, anaerobic treatment is followed by aerobic post-treatment. The suitability and the cost of the anaerobic-aerobic and aerobic treatment-systems are largely affected by a variety of mill-specific factors.