We review the theory of prewetting and wetting transitions in relation
to recent studies of noble gas and H-2 adsorption on alkali metal sur
faces. An essential feature is that the adsorption potentials have ext
remely small well depths. Open theoretical issues include the accuracy
of the potentials, the approximations used in the statistical mechani
cs, and dynamical effects of the substrate. New adsorbates and substra
tes are discussed.