Theories that invoke extra dimensions, such as string theory, typically give rise to many lower-dimensional vacuum states. For positive energy metastable minima, the cosmological implications of this picture are dramatic, leading to an eternally inflating universe which can seed many different vacuum phases. In this talk, I will highlight two ways that extra dimensions can play a key role in the dynamics of eternal inflation. First, I will describe how the stability of domain walls and vacuum bubbles separating four-dimensional vacua can be influenced by the properties of the overall volume modulus of a compactification. I will show that instabilities towards decompactification can prevent some vacua from being populated during eternal inflation. I will then outline a mechanism for dynamical compactification from de Sitter space, and describe various ``interpolating" solutions between spacetime regions with different numbers of non-compact dimensions.