  The basic theory of the linear and nonlinear ac susceptibilities of type-II
superconductors is outlined for various geometries, e.g. circular and elliptic
disks, rings, and strips of thin films or of thicker platelets in a
perpendicular magnetic field. It is shown how the linear susceptibility depends
on the linear complex resistivity or on the complex penetration depth, and the
nonlinear susceptibility on the nonlinear dependence of the electric field on
the current density and magnetic induction. The dependence of these
constitutive laws on the temperature and on various material properties like
pinning forces, pinning energies, structural defects, and granularity, leads to
an implicit dependence of the ac susceptibility on these parameters.
