  In the newly discovered iron-based superconductors, many experiments have
demonstrated the existence of the rotational symmetry breaking nematic order,
which has been a prevailing phenomenon in many correlated electronic systems.
In this paper, we review nematic behaviors in iron-pnictides and the mechanism
behind the development of the nematic order. We discuss evidences that support
the spin-driven nematicity in iron-pnictides. Theories, results and predictions
will be discussed based on this picture. We also briefly discuss the
generalization of this theory to the nematicity in iron-chalcogenides.
