  The realization of Dirac and Weyl physics in solids has made topological
materials one of the main focuses of condensed matter physics. Recently, the
topic of topological nodal line semimetals, materials in which Dirac or
Weyl-like crossings along special lines in momentum space create either a
closed ring or line of degeneracies, rather than discrete points, has become a
hot topic in topological quantum matter. Here we review the experimentally
confirmed and theoretically predicted topological nodal line semimetals,
focusing in particular on the symmetry protection mechanisms of the nodal lines
in various materials. Three different mechanisms: a combination of inversion
and time-reversal symmetry, mirror reflection symmetry, and non-symmorphic
symmetry, and their robustness under the effect of spin orbit coupling are
discussed. We also present a new Weyl nodal line material, the Te-square net
compound KCu$_2$EuTe$_4$, which has several Weyl nodal lines including one
extremely close to the Fermi level ($<$30 meV below E$_F$). Finally, we discuss
potential experimental signatures for observing exotic properties of nodal line
physics.
