  Magnetic fields are everywhere in the Universe and in our everyday life and
many processes are affected by their presence, generating a rich phenomenology
that depends also on other possible external agents. We review here some
results, both from our workgroup and from other research groups, about the
effect of magnetic fields on particles interaction processes, focusing mainly
on recent results, but without losing sight on early seminal works on this
topic. A vast assortment of physical situations and of analytical and numerical
approaches can be found in the literature in this subject, making the
comparison between them not straightforward. Our aim is to focus attention on
differences and similarities between the different situations and approaches,
looking for a systematization scheme that could be predictive, once the role
played by each physical ingredient could be understood. The main purpose of
this work is to find some physical explanations of the ongoing processes.
