Let $\Phi$ be an endomorphism of $\SR(\bar{\Q})$, the projective line over
the algebraic closure of $\Q$, of degree $\geq2$ defined over a number field
$K$. Let $v$ be a non-archimedean valuation of $K$. We say that $\Phi$ has
critically good reduction at $v$ if any pair of distinct ramification points of
$\Phi$ do not collide under reduction modulo $v$ and the same holds for any
pair of branch points. We say that $\Phi$ has simple good reduction at $v$ if
the map $\Phi_v$, the reduction of $\Phi$ modulo $v$, has the same degree of
$\Phi$.