We offer a pedagogical introduction to axion-like particles (ALPs) as far as
their relevance for high-energy astrophysics is concerned, from a few MeV to
1000 TeV. This review is self-contained, in such a way to be understandable
even to non-specialists. Among other things, we discuss two strong hints at a
specific ALP that emerge from two very different astrophysical situations. More
technical matters are contained in three Appendices.
