  Axion-like particles (ALPs) are light, neutral, pseudo-scalar bosons
predicted by several extensions of the Standard Model of particle physics --
such as the String Theory -- and are supposed to interact primarily only with
two photons. In the presence of an external magnetic field, photon-ALP
oscillations occur and can produce sizable astrophysical effects in the
very-high energy (VHE) band ($100 \, {\rm GeV} - 100 \, {\rm TeV}$). Photon-ALP
oscillations increase the transparency of the Universe to VHE photons partially
preventing the gamma-gamma absorption caused by the Extragalactic Background
Light (EBL). Moreover, they have important implications for active galactic
nuclei (AGN) by modifying their observed spectra both for flat spectrum radio
quasars (FSRQs) and BL Lacs. Many attempts have been made in order to constrain
the ALP parameter space by studying irregularities in spectra due to photon-ALP
conversion in galaxy clusters and the consequences of ALP emission by stars.
Upcoming new VHE photon detectors like CTA, HAWC, GAMMA-400, LHAASO,
TAIGA-HiSCORE, HERD and ASTRI will settle the issue.
