We share a common space-time with everything that we interact with in our world. An island universe would be a spatiotemporally interrelated segment of reality that is isolated from the rest of reality; it would be part of our world but something that we cannot interact with. Spatiotemporal interrelatedness plays an important role in a number of metaphysical theories concerning possible worlds. Here I discuss four problems surrounding the possibility of island universes. I contend the most troubling of these problems gives us good reason to think that island universes are possible; metaphysical theories that cannot make sense of the possibility of island universes end up having bizarre implications about alternate possible worlds.