In 1962, a drill probing for geothermal power at Niland, in the Imperial Valley of southern California, tapped what appeared to be an active ore solution (White et al, 1963). The mile-deep well discharged into a 275-foot long pipe and, after three months, the residue in this pipe amounted to more than five tons of dark, amorphous iron- and copper-rich material that assayed more than 381 ounces of silver and 0.11 ounces of gold per ton. Theorizing about the Niland brine, White (Roedder, 1965, p. 1394) suggests that the water is of meteoric origin and that during underground circulation it acquired its high salinity from evaporites enclosed in the rocks. This potent solution reacted with the rocks themselves at depths where the temperature was high, dissolved the iron, copper, silver and gold, carried them up, and finally deposited them, under unique circumstances, at the earths surface. Since even in this extraordinary case of ore deposition, the source of the metals and the origin of the brine is unknown, it is not surprising that the origin of natural ore deposits is the subject of many conflicting theories. Although there have been few fresh concepts in ore genesis theory over the past twenty years, the trend now is to admit several possible types of origin, depending on the deposit under consideration; not to be wedded to one theory only. Also more latitude is allowed for the origin of ore solutions and the source of the ore metals. Finally, possible mechanisms of ore transport are visualized in more detail than before. The rest of this paper will review briefly some of the more important concepts of ore genesis theory, illustrating them with examples. The general scheme will be to proceed from the more nearly understood aspects of ore deposition to the least understood. In line with this approach we turn first to the physico-chemical concepts of ore deposition.