  In this article, we consider fully nonlinear, possibly degenerate, parabolic
equations associated with Ventcell boundary conditions in bounded or unbounded,
smooth domains. We first analyze the exact form of such boundary conditions in
general domains in order that the notion of viscosity solutions make sense.
Then we prove general comparison results under natural assumptions on the
nonlinearities, assuming only that the equation is either coercive (first-order
case) or strictly elliptic (second-order case) in the normal direction in a
neighborhood of the boundary. Our method is inspired by the "twin blow-up
method" of Forcadel-Imbert-Monneau and ideas of Lions-Souganidis which we
extend to the framework of Ventcell boundary conditions.
