In the context of the probabilistic fine-tuning argument that moves from the fragility of cosmological parameters with respect to life to the existence of a divine designer, appealing to the existence of a multiverse has in general seemed problematically ad hoc. The situation looks rather different, though, if there is independent evidence from physics for a multiverse. I will argue that independently-motivated multiverses can be undercutting defeaters for the fine-tuning argument; but whether the argument is indeed undercut still depends on open questions in fundamental physics and cosmology. I will also argue that Everettian quantum mechanics opens up new routes to undercutting the fine-tuning argument, although by itself it is insufficient to do so.