  The recent discovery of topological insulator (TI) offers new opportunities
for the development of thermoelectrics, because many TIs (like Bi$_2$Te$_3$)
are excellent thermoelectric (TE) materials. In this review, we will first
describe the general TE properties of TIs and show that the coexistence of the
bulk and boundary states in TIs introduces unusual TE properties, including
strong size effects and anomalous Seebeck effect. Importantly, the TE figure of
merit $zT$ of TIs is no longer an intrinsic property, but depends strongly on
the geometric size. The geometric parameters of two-dimensional TIs can be
tuned to enhance $zT$ to be significantly greater than 1. Then a few
proof-of-principle experiments on three-dimensional TIs will be discussed,
which observed unconventional TE phenomena that are closely related to the
topological nature of the materials. However, current experiments indicate that
the metallic surface states, if their advantage of high mobility is not fully
utilized, would be detrimental to TE performance. Finally we provide an outlook
for future work on topological materials, which offers great possibilities to
discover exotic TE effects and may lead to significant breakthroughs in
improving $zT$.
