  We give field theory descriptions of the time-reversal invariant quantum spin
Hall insulator in 2+1 dimensions and the particle-hole symmetric insulator in
1+1 dimensions in terms of massive Dirac fermions. Integrating out the massive
fermions we obtain a low-energy description in terms of a topological field
theory, which is entirely determined by anomaly considerations. This
description allows us to easily construct low-energy effective actions for the
corresponding `fractional' topological insulators, potentially corresponding to
new states of matter. We give a holographic realization of these fractional
states in terms of a probe brane system, verifying that the expected
topologically protected transport properties are robust even at strong
coupling.
