  The relaxation properties of dielectric materials are described, in the
frequency domain, according to one of the several models proposed over the
years: Kohlrausch-Williams-Watts, Cole-Cole, Cole-Davidson, Havriliak-Negami
(with its modified version) and Excess wing model are among the most famous.
Their description in the time domain involves some mathematical functions whose
knowledge is of fundamental importance for a full understanding of the models.
In this work, we survey the main dielectric models and we illustrate the
corresponding time-domain functions. In particular, we stress the attention on
the completely monotone character of the relaxation and response functions. We
also provide a characterization of the models in terms of differential
operators of fractional order.
