  The newly discovered iron-based superconductors have triggered renewed
enormous research interest in the condensed matter physics community. Nuclear
magnetic resonance (NMR) is a low-energy local probe for studying strongly
correlated electrons, and particularly important for high-$T_{\rm C}$
superconductors. In this paper, we review NMR studies on the structural
transition, antiferromagnetic order, spin fluctuations, and superconducting
properties of several iron-based high-$T_{\rm C}$ superconductors, including
LaFeAsO$_{1-x}$F$_x$, LaFeAsO$_{1-x}$, BaFe$_2$As$_2$,
Ba$_{1-x}$K$_x$Fe$_2$As$_2$, Ca$_{0.23}$Na$_{0.67}$Fe$_2$As$_2$,
BaFe$_2$(As$_{1-x}$P$_x$)$_2$, Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$,
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, Li$_{1+x}$FeAs, LiFe$_{1-x}$Co$_x$As, NaFeAs,
NaFe$_{1-x}$Co$_x$As, K$_y$Fe$_{2-x}$Se$_2$, and (Tl,Rb)$_y$Fe$_{2-x}$Se$_2$.
