Age dating of ground water from crystalline rocks, shales, and clayey sediments is an essential component in assessing hydrogeologic suitability of such rocks for long-term containment of toxic materials. Isotope techniques available for this task are discussed and it is noted that, at present, only three tools can be used for these investigations. They are based on tritium, 14C, and elements of the uranium decay series. There is little hope that others will be developed in the near future except for applications where it is possible to collect extremely large volumes of water within a reasonable time. The limitations of the available tools are discussed and directions in which major research efforts should proceed in future are indicated. It is also emphasized that no single tool, including isotope analyses, can provide absolute water ages. Therefore, these techniques must be combined with geochemical, hydrological, and mathematical studies before it will be possible to arrive at reasonable age estimates of ground waters in low-permeability rocks.