An integrable system is a Hamiltonian dynamical system with maximum number of symmetries. For instance, the spherical pendulum, with time translation and rotation about the vertical axis as the symmetries, is an integrable system. The fixed point, being still at the top, is called a singularity of focus-focus type. We will define and give the local model of focus-focus singular points. Then we give the symplectic classification of the germs of integrable systems at fibers with focus-focus singularities.