Under anaerobic conditions bacteria possessing specialized respiratory pathways can use arsenic (As) and selenium (Se) to generate cellular energy and sustain cell growth and survival. As and Se share common chemical properties and have a complex relationship with bacteria which act as a tremendous vector in the biogeochemical cycling of the two elements. Bacteria generate energy (ATP) through respiration by coupling the oxidation of an electron donor (organic or inorganic) with the reduction of the electron acceptor (in this case, certain oxyanions of arsenic and selenium). Interestingly, certain bacterial strains can utilize over 20 combinations of e-donors and e-acceptors, thus showing broad range metabolic repertoire and complex evolutionary scenarios. In contrast, certain strains only use a limited range of compounds for respiratory processes, being restricted to environments with well-defined characteristics. The use of As and Se compounds for anaerobic respiration in bacteria started to be acknowledged at the end of the 1980s, while the full exploration of such processes (thermodynamics, enzymology and genetic determinants) emerged in the last decade of the 20th century and continues to this day. Both arsenic and selenium respiration appear to have had a major importance for bacteria during the ancient geologic periods when oxygen was present in very low concentrations in the atmosphere. Contemporarily, certain geological situations and terrestrial and aquatic ecosystems favor the presence of As and Se in considerable concentrations. In such contexts, bacteria still rely heavily of these elements to ensure proper metabolic functions, growth, and reproduction, and, in turn, contribute to the sustainability of such fragile ecosystems.