The quantum theory of particles which is consistent with the special theory of relativity, and thus can describe particles moving arbitrarily close to the speed of light. It is now realized that the only satisfactory relativistic quantum theory is quantum field theory; the attempt to relativize the Schrödinger equation for the wave function of a single particle fails, as described below. However, with a change of interpretation, relativistic wave equations do correctly describe some aspects of the motions of particles in an electromagnetic field.