: The input-to-state stability (ISS) problem is studied for switched systems with infinite subsystems. By the method of multiple Lyapunov function, a sufficient ISS condition is given based on a quantitative relation of the input and the values of the Lyapunov functions of the subsystems before and after the switching instants. In terms of the average dwell-time of the switching laws, some sufficient ISS conditions are obtained for switched nonlinear systems and switched linear systems, respectively. Copyright c 2005 IFAC