Very light axion-like particles (ALPs) with a two-photon vertex are predicted in many extensions of the Standard Model of particle physics. Depending on the actual value of their mass, ALPs can play an important role in cosmology, acting as cold dark matter. The coupling with photons allows for ALP-photon mixing in external electromagnetic fields. This effect is exploited for direct searches of ALPs in laboratory experiments. The two-photon vertex would also induce the mixing with ALPs for photons emitted by distant astrophysical sources, and propagating in the large-scale cosmic magnetic fields. This mixing could produce signatures in astrophysical observations, ranging from the cosmic microwave background to the high-energy gamma-rays. In this talk I will present an overview of current bounds on ALPs and discovery opportunities in the planned laboratory and astrophysical experiments.