%0 Journal Article %T Problems with the Klein-Gordon Theory %A Eliahu Comay %J Open Access Library Journal %V 10 %N 8 %P 1-12 %@ 2333-9721 %D 2023 %I Open Access Library %R 10.4236/oalib.1110511 %X The electromagnetic domain comprises two kinds of physical objects¡ªelectromagnetic fields and electrically charged particles. Therefore, the structure of a comprehensive electromagnetic theory is a coherent union of two theories. One theory describes electromagnetic fields, and the second theory describes electrically charged particles. An obvious requirement says that a comprehensive electromagnetic theory must be a coherent union of a theory of electromagnetic fields and a theory of electrically charged particles. The continuity equation is a well-known example showing how Maxwell equations of the electromagnetic fields impose a constraint on a theory that describes the time evolution of a charged particle. The novelty of this work is its proof that the continuity equation is not a unique example. Namely, the Maxwell theory of electromagnetic fields imposes other constraints on a theory of an electric charge. This work shows that the classical theory as well as the Dirac theory of a spin-1/2 charged quantum particle provide a coherent electromagnetic theory. In contrast, new problems arise in the Klein-Gordon theory of a charged spin-0 quantum particle. %K Classical Electrodynamics %K Dirac Theory %K Klein-Gordon Theory %K The Continuity Equation %U http://www.oalib.com/paper/6801118