  In the last years it has been realized, that extensions of the Standard Model
may manifest itself also at meV energy scales. The low energy frontier is a
rich complement to the conventional high-energy particle physics landscape. The
search for these new particles initiated experimental activities around the
world. "Light shining through a wall" (LSW) experiments search for Weakly
Interacting Sub-eV Particles (WISP). Potential WISP candidates are axion-like
particles or hidden sector photons. The ALPS (Any Light Particle Search)
experiment located at DESY in Hamburg exploits resonant laser power build-up in
a large-scale optical cavity to boost the available power for the WISP
production. After some upgrades the experiment provides now the most stringent
laboratory constraints on WISP production. The concept, challenges and status
of LSW experiments as well as their future potential are presented.
