We calculate the friction force between two semi-infinite solids in relativ
e parallel motion (with velocity V), and separated by a vacuum gap of width
d. The friction force results from coupling via a fluctuating electromagne
tic field, and can be considered as the dissipative part of the van der Waa
ls interaction. We consider the dependence of the friction force on the tem
perature T, and present a detailed discussion of the limiting cases of smal
l and large V and d.