The grain size dependence of the "giant dielectric effect" in CaCu3Ti4O12 ceramics (arising from a semiconducting grain/insulating grain boundary microstructure) has been revealed by impedance spectroscopy of ceramics sintered for 3 h (see Figure, left) and 24 It (see Figure, right); a rise in permittivity from 9000 to 280000 was found. The easy-to-prepare CaCu3Ti4O12 is a hot candidate for ceramic capacitor applications.