  We propose a theory which allow to calculate the magnetic excitation spectrum
in diluted ferromagnetic systems. The approach is rather general and based on
the Self-Consistent local Random Phase Approximation in which disorder
(dilution) and thermal fluctuations are properly treated. To illustrate its
reliability and accuracy we calculate the magnetic excitation in the diluted
III-V magnetic semiconductor $Ga_{1-x}Mn_{x}As$. It is shown that dilution has
a drastic effect on the excitation spectrum, indeed well defined magnon
excitations exist only in a small region of the Brillouin zone centered around
the $\Gamma$ point. We also calculate the spin stiffness in optimally annealed
sample as a function of $Mn^{2+}$ concentration. A comparison to available
measurements is done. We find a very good agreement for both the Curie
temperature and the spin stiffness measured in well annealed samples and
provide a plausible explanation for the very small values measured in as grown
samples.
