  Topological insulators are new states of quantum matter with surface states
protected by the time-reversal symmetry. In this work, we perform
first-principle electronic structure calculations for $Sb_2Te_3$, $Sb_2Se_3$,
$Bi_2Te_3$ and $Bi_2Se_3$ crystals. Our calculations predict that $Sb_2Te_3$,
$Bi_2Te_3$ and $Bi_2Se_3$ are topological insulators, while $Sb_2Se_3$ is not.
In particular, $Bi_2Se_3$ has a topologically non-trivial energy gap of $0.3
eV$, suitable for room temperature applications. We present a simple and
unified continuum model which captures the salient topological features of this
class of materials. These topological insulators have robust surface states
consisting of a single Dirac cone at the $\Gamma$ point.
