  Axions and other very weakly interacting slim particles (WISPs) may be
non-thermally produced in the early universe and survive as constituents of the
dark universe. We describe their theoretical motivation and their
phenomenology. A huge region in parameter space spanned by their couplings to
photons and their masses can give rise to the observed cold dark matter
abundance. A wide range of experiments - direct dark matter searches exploiting
microwave cavities, searches for solar axions or WISPs, and
light-shining-through-a-wall searches - can probe large parts of this parameter
space in the foreseeable future.
