The classical z-transform is not adequate for calculations on time-varying systems, it assumes that the system under consideration is linear time-invariant. The most successful attempt at extension of the z-transform to linear time-varying systems has been called the 'W-transform'. It is a straight generalization of the z-transform and it remarkably succeeds in providing a framework in which most of classical linear time-invariant synthesis techniques also extend to the time-varying case. The lecture gives the definition of the W-transformation and one typical application of it, interpolation.