  We present a critical review and summary of String Gas Cosmology. We include
a pedagogical derivation of the effective action starting from string theory,
emphasizing the necessary approximations that must be invoked. Working in the
effective theory, we demonstrate that at late-times it is not possible to
stabilize the extra dimensions by a gas of massive string winding modes. We
then consider additional string gases that contain so-called enhanced symmetry
states. These string gases are very heavy initially, but drive the moduli to
locations that minimize the energy and pressure of the gas. We consider both
classical and quantum gas dynamics, where in the former the validity of the
theory is questionable and some fine-tuning is required, but in the latter we
find a consistent and promising stabilization mechanism that is valid at
late-times. In addition, we find that string gases provide a framework to
explore dark matter, presenting alternatives to $\Lambda$CDM as recently
considered by Gubser and Peebles. We also discuss quantum trapping with string
gases as a method for including dynamics on the string landscape.
