In this paper we present a general theoretical framework from which several known results (a some new ones) on the existence and stability of solitons can be recovered. We give an abstract definition of solitary wave and soliton and we develope an abstract existence theory. This theory provides a powerful tool to study the existence of solitons for the Klein-Gordon equations as well as for gauge theories. Applying this theory, we prove the existence of a continuous family of stable charged Q-balls