Water, through its unique and extreme properties, is the fundamental fluid genetically relating all mineral deposits in sedimentary rocks. Economically important mineral deposits in sedimentary rocks which are the result of natural water-rock interaction include petroleum and Mississippi-type lead-zinc deposits. Understanding of the origin of these deposits through water-rock interaction requires knowledge of the relations between hydrochemistry and hydrodynamics. The recovery of some of these mineral deposits involves man-imposed water-rock interactions, for example, during water flooding of petroleum reservoirs, in-situ steam injection into oil sand deposits and underground coal gasification. These man-imposed water-rock interactions may result in subsurface reactions w ich can reduce permeability, produce toxic or deleterious substances which require removal before reuse of the produced water, contaminate local potable groundwater, or cause problems in waste injection wells because of subsequent water-rock reactions. Although we understand some of the principles involved, it is clear that considerably more thought and additional research effort needs to be directed to these and other economic aspects of water-rock interaction.