The term bioleaching refers to the conversion of an insoluble metal (generally a metal sulfide, e.g. CuS, NiS, ZnS) into a soluble form (usually the metal sulfate, e.g. CuSO4, NiSO4, ZnSO4). When the metal is extracted into water; this process is called bioleaching. The term bio-oxidation is usually used to refer to processes in which the recovery of a metal is enhanced by microbial decomposition of the mineral, but the metal being recovered is not solubilized. Bio-mining is a general term that may be used to refer to both processes. Bacterial leaching of metals is a process in which the ores of the metals, usually their sulfides, are solubilized by bacterial action. Bacterial leaching has been practiced by man over many centuries without any understanding of the microbiological basis of the process. Indeed, about 12% of copper produced in the USA is obtained by bacterial leaching of low grade ores. This chapter describes commercial leaching methods by irrigation-type processes and stirred tank processes, microbiology of the leaching process, leaching of some metal sulfides, environmental conditions affecting bacterial leaching.