The main factors responsible for anaerobic recalcitrance are reviewed. Anaerobic recalcitrance is associated with hydrocarbons lacking functional groups, branched molecules (gasoline oxygenates), aromatic amines and aromatic sulfonates. The most recalcitrant compounds are high molecular weight non-hydrolyzable polymers such as plastic, lignin and humus, which cannot be taken up by cells. Recently new capabilities of anaerobic microorganisms have been discovered to degrade compounds previously considered to be recalcitrant. For example, anaerobic bacteria initiate the degradation of alkylbenzenes and alkanes with an unusual addition reaction with fumarate, forming a hydrocarbon-succinate adduct. Finally, new evidence indicates that the most recalcitrant compounds (humic substances) are not so inert and can play important roles in aiding the biodegradation of other compounds by serving as an electron acceptor or redox mediator.