KEK’s accelerators must be on the frontier of the performance with state-of-art technologies. Accelerator theory gives the basis of such technologies, covering beam dynamics for single and collective particle motions, electromagnetic waves and radiation, interactions with matters, plasma and beams. Both analytical and computational methods are necessary to carry out the mission. Heavy commitments in all accelerators and beams at KEK are necessary. International collaboration is another key of the development of accelerator theory.