Propagating gamma-ray photons from Galactic and extragalactic sources can mix in the pres- ence of magnetic fields with light pseudo-scalar particles that are predicted in many extension of standard model physics. Even though the magnetic fields in astrophysical environments are small compared to values achievable in the laboratory, the large distances compensate this to saturate the conversion probabilities even for couplings which are below the value probed in laboratory exper- iments. There are in principle, two different observational tests for the presence of new and light (typically with masses of neV or below) pseudo scalar particles possible: either the disappear- ance of propagating photons due to conversion of photons to pseudo-scalars or the appearance of photons converting and re-converting after propagating through an optically thick medium (e.g., gamma-ray sources at cosmological distance). In this contribution, the two observational tests are described and available observations summarized.