Klein Gordon equation is a useful dynamical equation for a certain class of particles. A problem arises however in the probabilistic interpretation of its solutions as representing a single particle. The difficulty with the Klein Gordon equation is that it has both positive and negative energy solutions. This can be shown to give rise to antiparticles which must be included for self consistency. The Klein Gordon equation can be used to solve the hydrogen atom problem in a relativistically correct way; it is only appropriate for spin-0 particles, and thus does not apply directly to electrons and the real hydrogen atom. It is useful for pionic (π) and kaonic (K) atoms, and does give some idea of the form of the relativistic corrections.