The nature of the glass transition in liquids composed of monatomic species is discussed using a hard sphere model. Recent molecular dynamics calculations by Woodcock [J. Chem. Soc. Faraday II 72, 1667 (1976)] indicate that a hard sphere fluid undergoes a glass transition when it is compressed to a high enough density. This calculation provides one of several independent methods of estimating the packing fraction at the glass transition, ηg, for hard spheres. The estimated value of ηg for hard spheres, 0.533 ±0.014, is substantially lower than 0.637, the packing fraction of dense random packed hard spheres. The effective ηg for a number of atomic glass forming liquids with continuous interatomic potentials also are estimated, and are found to lie in or near the range of estimated values of ηg for hard spheres. The liquids considered are a Lennard-Jones fluid for which the liquid–glass transition has been studied by a molecular dynamics calculation [A. Rahman, M. Mandell, and J. McTague, J. Chem. Phys. 64, 1564 (1976)], and several real glass forming metal–metalloid alloys. The similarity of ηg for the hard sphere fluid, for the Lennard-Jones fluid, and for metal alloys suggest that it is the short ranged repulsive forces acting between atoms which are responsible for the glass transition. These results also suggest that in general, ηg=0.53±0.02 for atomic liquids. This provides a criterion for predicting the glass transition temperatures for materials which have not yet been observed in the glassy state.