  Recently the PVLAS collaboration reported the observation of a rotation of
linearly polarized laser light induced by a transverse magnetic field -- a
signal being unexpected within standard QED. In this review, we emphasize two
mechanisms which have been proposed to explain this result: production of a
single light neutral spin-zero particle or pair production of light minicharged
particles. We discuss a class of models, involving, in addition to our familiar
``visible'' photon, further light ``hidden paraphotons'', which mix
kinematically with the visible one, and further light paracharged particles. In
these models, very strong astrophysical and cosmological bounds on the weakly
interacting light particles mentioned above can be evaded. In the upcoming
year, a number of decisive laboratory based tests of the particle
interpretation of the PVLAS anomaly will be done. More generally, such
experiments, exploiting high fluxes of low-energy photons and/or large
electromagnetic fields, will dig into previously unconstrained parameter space
of the above mentioned models.
