The use of fission fragment damage tracks for dating of geological and other glasses is described and applied to a number of tektites and a Libyan glass. Most of the tektite dates are in good agreement with those established by K-Ar measurements. For Libyan glass the fission track age is 34 ± 6 million years. Elevated temperature experiments indicate that tracks in tektites would be stable at 22°C for much longer than the age of the earth and could disappear within 100 million years only if the temperature of the glass had exceeded 170°C. A suggestion for a method of dating thermal histories is presented.