I will discuss recent work in brane gas cosmology, and the role of winding modes in determining the effective dimensionality of the universe at late times. Winding modes exist in any cosmological model where the spatial hypersurfaces have a suitable topology and the fundamental constituents of matter are extended objects — either strings or branes. This idea is due to Brandenberger and Vafa (1989) who argued that strings wrapped round a toroidal spacetime could naturally lead to a universe with three "large" spatial directions. I will make a brief digression to describe the extension of the Brandenberger-Vafa hypothesis to orbifold spacetimes, and then focus on the dynamics of a universe inspired by the low energy limit of M-theory, where 2-branes and a supergravity gas exist inside a universe whose spatial sections are 10 dimensional torii. I will first show that the possible late-time asymptotics for this universe can be simply categorized, and that the numbers of large spatial directions are determined by the wrapping modes of the 2-branes. I will then describe new work on the thermodynamics of the branes and supergravity gas, and the role of thermal fluctuations in selecting different possible asymptotic solutions, and thus the number of macroscopic spatial directions that exist at late times.