Axions are hypothetical particles that may explain the observed dark matter density and the non-observation of a neutron electric dipole moment. If the axion emerges after inflation, topological defects called axion strings are created in the early Universe. The dynamics of those axion strings are non-linear and complex but understanding them allows us to make concrete predictions such as the value of the QCD axion mass. In this talk, I will comment on some of the difficulties that arise when studying axion strings and the possible observational signatures of axion strings in ALP scenarios.