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I will first review basics aspects of axion like particles and their role in beyond the standard model physics. I will then discuss the appearance of topological defects, specifically cosmic strings and domain walls, in the cosmological history of models with axion like particles. These axionic defects have a rich phenomenology which I will illustrate: they can produce dark matter in the early Universe, they can be seeds for a first order electroweak phase transitions, they can generate the matter/anti-matter asymmetry, and furthermore they are powerful sources of a background of gravitational waves which will be probed in current and future gravitational wave experiments.