  It has been notoriously difficult to construct a meta-stable de Sitter (dS)
vacuum in string theory in a controlled approximation. This suggests the
possibility that meta-stable dS belongs to the swampland. In this paper, we
propose a swampland criterion in the form of $|\nabla V|\geq\ c \cdot V$ for a
scalar potential $V$ of any consistent theory of quantum gravity, for a
positive constant $c$. In particular, this bound forbids dS vacua. The
existence of this bound is motivated by the abundance of string theory
constructions and no-go theorems which exhibit this behavior. We also extend
some of the well-known no-go theorems for the existence of dS vacua in string
theory to more general accelerating universes and reinterpret the results in
terms of restrictions on allowed scalar potentials.
