Some leading cosmologists argue that it is no longer a question of whether other universes exist. It is simply a question of how many different levels there are to the so-called Multiverse. For example, almost all working cosmologists already accept the existence of regions of space beyond the accessible and observable part of the universe. These comprise distant regions which are so far apart from us that we could never reach them, even if we traveled at the speed of light forever, provided that general relativity is correct and that the expansion of our universe keeps accelerating forever. Other, more speculative, levels of the Multiverse have been discussed that are nonetheless straightforward logical extrapolations of our best theories of physics, which have been extremely well tested in other regimes. However, it remains a significant challenge to determine how to test theories that predict various types of Multiverse, especially because they might include regions that seem to be unobservable, in principle. This controversial topic thus represents an exciting area at the leading edge of theoretical physics.