  In this paper we study the generalized mean curvature flow of sets in the
sub-Riemannian geometry of Carnot groups. We extend to our context the level
sets method and the weak (viscosity) solutions introduced in the Euclidean
setting by Evans-Spruck and Chen-Giga-Goto. We establish two special cases of
the comparison principle, existence, uniqueness and basic geometric properties
of the flow.
