  Most embeddings of the Standard Model into a more unified theory, in
particular the ones based on supergravity or superstrings, predict the
existence of a hidden sector of particles which have only very weak
interactions with the visible sector Standard Model particles. Some of these
exotic particle candidates (such as e.g. "axions", "axion-like particles" and
"hidden U(1) gauge bosons") may be very light, with masses in the sub-eV range,
and have very weak interactions with photons. Correspondingly, these very
weakly interacting sub-eV particles (WISPs) may lead to observable effects in
experiments (as well as in astrophysical and cosmological observations)
searching for light shining through a wall, for changes in laser polarisation,
for non-linear processes in large electromagnetic fields and for deviations
from Coulomb's law. We present the physics case and a status report of this
emerging low-energy frontier of fundamental physics.
