  For any integral lattice $Q$, one can construct a vertex algebra $V_Q$ called
a lattice vertex algebra. If $\sigma$ is an automorphism of $Q$ of finite
order, it can be lifted to an automorphism of $V_Q$. In this paper we classify
the irreducible $\sigma$-twisted $V_Q$-modules. We show that the category of
$\sigma$-twisted $V_Q$-modules is a semisimple abelian category with finitely
many isomorphism classes of simple objects.
