The ITER project is an important step toward fusion energy utilization for human beings. Fundamental understanding of physics is quite important, as well as understanding of tokamak systems and plasma control. In this lecture, given at the 4th ITER International Summer School, we give an introduction to tokamak research on fusion energy and ITER in light of the main theme of this school, plasma control, and we present some examples to illustrate the importance of physics by showing some physics elements underlying research toward steady-state operation of reactor-relevant tokamak plasmas.