  We present some bottom-up motivations of axions and other weakly interacting
sub-eV particles (WISPs) coupling to photons. Typically, these light particles
are strongly constrained by their production or interaction in astrophysical
and cosmological environments. Dedicated laboratory searches can provide
complementary probes that are mostly less sensitive but also less
model-dependent. We briefly comment on future experiments with the potential to
discover photon oscillation effects in kinetic mixing scenarios with massive
hidden photons.
