  We calculate the friction force between two semi-infinite solids in relative
parallel motion (velocity $V$), and separated by a vacuum gap of width $d$. The
friction force result from coupling via a fluctuating electromagnetic field,
and can be considered as the dissipative part of the van der Waals interaction.
We consider the dependence of the friction force on the temperature $T$, and
present a detailed discussion of the limiting cases of small and large $V$ and
$d$.
