  We study the stability of quantum motion of classically regular systems in
presence of small perturbations. Onthe base of a uniform semiclassical theory
we derive the fidelity decay which displays a quite complexbehaviour, from
Gaussian to power law decay $t^{-\alpha}$ with $1 \le \alpha \le 2$.
Semiclassical estimates are given for the time scales separating the different
decaying regions and numerical results are presented which confirm our
theoretical predictions.
