Aromatic compounds such as aromatic amines (AAs) or lignocellulose biomass derivatives are often recalcitrant to biological processes (i.e., anaerobic digestion). The level of recalcitrance is usually explained by the biochemical structure of the compounds, which determines their removal. AAs are present in dye wastewaters mainly originated from textile industries and are considered carcinogenic and challenging pollutants to be removed from wastewater. The hydrolysis of lignin leads to the formation of some aromatic compounds, which could be recalcitrant at certain concentrations and operation conditions. Similarly, derivatives from sugar degradation such as furfurals and 5-hydroxymethylfurfural (HMF) from pentose and hexose degradation, respectively, have also been reported to be recalcitrant to anaerobic treatment. This chapter details the different forms of recalcitrance found and/or formed during wastewater and organic solid waste treatment, particularly in the anaerobic digestion process. Moreover, the possible strategies to mitigate the recalcitrance of these compounds are comprehensively explained.@en