The anthropocentric view is that oxygen is essential for life, ranging from the use of oxygen as an electron acceptor for respiratory oxidases to oxygenases that can attack and biologically degrade almost any chemical compound. However, it is now generally accepted that anaerobic bacteria are capable of carrying out almost all biological processes without oxygen and are key players in global elemental cycles or in the removal of environmentally relevant toxic pollutants. Anaerobic bacteria use oxygen-independent biocatalysts, many of which have only been identified and studied in detail in the last decade. For example, they play a key role in the global elimination of toxic or carcinogenic aromatics and steroids, or in the conversion of biomass to biogas. The underlying processes often follow novel bioenergetic principles and mechanisms achieved through the use of unprecedented cofactors. Some of the novel biocatalysts are being evaluated for biotechnological applications or have led to the discovery of novel drug targets. The ongoing discovery of new anaerobic metabolic capabilities in the growing number of unculturable anaerobic microorganisms points to further applications in the future.