  We review how axion-like particles (ALPs) naturally emerge in the low-energy
effective field theory of string compactifications. We focus on the study of
their mass spectrum and couplings, stressing that they depend on the mechanism
used to fix the moduli. We present concrete examples where either open or
closed string modes behave as QCD axions which do not overproduce cold dark
matter. Relativistic ALPs can also be produced by the decay of the lightest
modulus which drives reheating. These ALPs contribute to dark radiation and
could be detected via axion-photon conversion in astrophysical magnetic fields.
