A new model describes charge and mass transfer in ice-ice collisions in terms of fundamental molecular physics. Drawing on clues from results of recent and earlier experiments, the theory treats the collisions as interlinked events acting in three stages: before collision, during contact, and withdrawal. The theory provides quantitative descriptions of charge and mass transfer and their dependence on growth rate, temperature, and impact energy. Application of the theory to experimental simulations of thunderstorm charging explains general trends in terms of basic microscopic processes. ¿ 2001 American Geophysical Union