Suspended and attached growth wastewater treatment processes are widely used for the removal of biodegradable organic constituents and nutrients from wastewater. The frequent detection of toxic metals and metalloids in wastewater contributed to the publishing of numerous papers in the past, investigating their presence, toxicity and behavior in biological wastewater treatment systems (BWTS). In most of these studies, data were referred to total element concentration. However, nowadays it is generally accepted that toxicological and physicochemical properties are differentiated according to elemental speciation. The literature review revealed that there are enough studies investigating chromium species presence, toxicity, fate and biotransformation in BWTS, whereas there are fewer data for organotin compounds. Even less studies exist for arsenic, mercury and selenium species. Most of the results are referred to the activated sludge process. The biotransformation mechanisms are element-dependent. The reduction of Cr(VI) to Cr(III), the sequential debutylation of tributyltin and triphenyltin to inorganic tin, the oxidation of As(III) to As(V) and their methylation to organoarsenicals have been observed. Despite the gaps in the literature, these preliminary, in many cases, results indicate that biological wastewater treatment processes could be used in the future for the detoxification of metal-and metalloid-laden wastewater, allowing the reuse of wastewater and sludge.