  Roughening of interfaces implies the divergence of the interface width $w$
with the system size $L$. For two-dimensional systems the divergence of $w^2$
is linear in $L$. In the framework of a detailed capillary wave approximation
and of statistical field theory we derive an expression for the asymptotic
behaviour of $w^2$, which differs from results in the literature. It is
confirmed by Monte Carlo simulations.
