The macroscopic behaviour of superconductors is well described by London equations, which provide an explanation for both the zero DC resistance and the Meissner effects. In thin films, the traditional London penetration depth which accounts for the current and magnetic field distribution in bulk superconductors, is replaced by a two-dimensional screening length called the Pearl length. The penetration depth is linked to another property of superconducting materials, the kinetic inductance, which is of critical importance in the design of detectors and electrical circuits. The current that can be carried by superconductors is limited to a value called critical current, above which the sample becomes resistive.