I will discuss the role of string gases in early universe cosmology.
We will see that the 1-loop free energy for a string gas at finitie
temperature will be enough
to stabilize many of the moduli from the ten dimensional perspective.
At late times such stabilization mechanisms will generically
fail and the moduli will begin to roll, but a natural
hierarchy of scales will remain.  We will see that it is possible to
regain stabilization by invoking the idea of moduli trapping, which
results from considering the
consequences of particle production in the time-dependent background.
This also leads to exciting new possibilities for realizing dark energy
and dark matter in the context of string theory model building.