  In this paper we review recent results on the existence of non-topological
solitons in classical relativistic nonlinear field theories. We follow the
Coleman approach, which is based on the existence of two conservation laws,
energy and charge. In particular we show that under mild assumptions on the
nonlinear term it is possible to prove the existence of solitons for a set of
admissible charges. This set has been studied for the nonlinear Klein-Gordon
equation, and in this paper we state new results in this direction for the
Klein-Gordon-Maxwell system.
