Abstract: (Which topics? The interesting ones!) Light pseudo-Goldstone bosons arise in many compelling models of particle physics and string theory. These particles can modify cosmology in interesting, surprising, and testable ways. In this talk I will discuss three topics. First I’ll argue that the dynamics of an axion field during inflation can give rise to the matter / antimatter asymmetry of the universe via the production of helical magnetic fields (which persist today and might be detectable!). Second, I’ll discuss what goes wrong when you try to implement the same idea with chiral gravitational waves instead of magnetic fields. Finally, I’ll talk about new phenomena that arise in multi-axion models, such as the recent proposed `clockwork’ axion, that have a large hierarchy between the scale of PQ-breaking and the axion decay constant.