  Shining light through walls? At first glance this sounds crazy. However, very
feeble gravitational and electroweak effects allow for this exotic possibility.
Unfortunately, with present and near future technologies the opportunity to
observe light shining through walls via these effects is completely out of
question. Nevertheless there are quite a number of experimental collaborations
around the globe involved in this quest. Why are they doing it? Are there
additional ways of sending photons through opaque matter? Indeed, various
extensions of the standard model of particle physics predict the existence of
new particles called WISPs - extremely weakly interacting slim particles.
Photons can convert into these hypothetical particles, which have no problems
to penetrate very dense materials, and these can reconvert into photons after
their passage - as if light was effectively traversing walls. We review this
exciting field of research, describing the most important WISPs, the present
and future experiments, the indirect hints from astrophysics and cosmology
pointing to the existence of WISPs, and finally outlining the consequences that
the discovery of WISPs would have.
