Investigation of the role of ionization in the Earth’s atmosphere and its applications is a multidisciplinary problem that requires a collaboration of specialists looking at the problem from different points of view. Here we present an overview of the ISSI Team project where we aim to increase and synthesize the understanding of ionization sources in the Earth’s atmosphere, in particular the parameterization of ionization rates by middle range energies and relativistic electrons, whose atmospheric impact is still to a large extent unknown. We see two clear strands of work, one concerned with atmospheric measurements of ionization, and the other with the parameterization the ionization rates and their influence on atmospheric processes. These two streams are, of course, complimentary and inter-related.