A review of the growth requirements for microorganisms and biodeterioration in geothermal energy systems. Geothermal waters usually contain many salts, often in varying concns. Some of these salts, esp. if they are oxidizable or reducible, may be subject to microbial conversion and/or (bio)pptn. Microorganisms can oxidize, even under anoxic conditions, reduced S compds., Fe(II) ions, Mn(II) ions, etc. However, partially or fully oxidized compds. can be reduced by microorganisms, for example S compds., Fe(III) ions, Mn(IV) ions, N oxides such as nitrite and nitrate, and also bicarbonate and carbonate ions. If org. compds. are present they may be oxidized or reduced. A no. of these microorganisms are able to perform such a metab. under aerobic or anoxic conditions. All these (bio)processes allow bacteria to grow and proliferate. The consequences include biocorrosion and biodeterioration.