  The aim of this tutorial survey is to revisit the basic theory of relaxation
processes governed by linear differential equations of fractional order. The
fractional derivatives are intended both in the Rieamann-Liouville sense and in
the Caputo sense. After giving a necessary outline of the classical theory of
linear viscoelasticity, we contrast these two types of fractional derivatives
in their ability to take into account initial conditions in the constitutive
equations of fractional order. We also provide historical notes on the origins
of the Caputo derivative and on the use of fractional calculus in
viscoelasticity.
