  In the present work we define and study the classifying (or "quotient") site
$[X/\Sigma]$ for any small site $X$ with (countable) coproducts endowed with an
action of a (countable) semigroup $\Sigma$. A simple case (the most relevant to
our applications) is the case $\Sigma=\mathbb{N}$, on which, therefore we
concentrate. Our main result consists in establishing an equivalence of the
corresponding T\`opos with the category of sheaves on $X$ with
``$\Sigma-$action''. We prove also that there is a spectral sequence computing
sheaf cohomology in $[X/\mathbb{N}]$ and we deduce some topological properties
of this site, such as its fundamental group. We finally apply the above
formalism in Holomorphic Dynamics, giving a T\`opos-theoretic interpretation of
Epstein's work on the Fatou-Shishikura Inequality and Infinitesimal Thurston's
Rigidity.
